Nim/compiler/sigmatch.nim

#
#
#           The Nim Compiler
#        (c) Copyright 2013 Andreas Rumpf
#
#    See the file "copying.txt", included in this
#    distribution, for details about the copyright.
#

## This module implements the signature matching for resolving
## the call to overloaded procs, generic procs and operators.

import
  intsets, ast, astalgo, semdata, types, msgs, renderer, lookups, semtypinst,
  magicsys, condsyms, idents, lexer, options, parampatterns, strutils, trees,
  nimfix / pretty, lineinfos

when not defined(noDocgen):
  import docgen

type
  TCandidateState* = enum
    csEmpty, csMatch, csNoMatch

  CandidateError* = object
    sym*: PSym
    unmatchedVarParam*, firstMismatch*: int
    diagnostics*: seq[string]
    enabled*: bool

  CandidateErrors* = seq[CandidateError]

  TCandidate* = object
    c*: PContext
    exactMatches*: int       # also misused to prefer iters over procs
    genericMatches: int      # also misused to prefer constraints
    subtypeMatches: int
    intConvMatches: int      # conversions to int are not as expensive
    convMatches: int
    state*: TCandidateState
    callee*: PType           # may not be nil!
    calleeSym*: PSym         # may be nil
    calleeScope*: int        # scope depth:
                             # is this a top-level symbol or a nested proc?
    call*: PNode             # modified call
    bindings*: TIdTable      # maps types to types
    magic*: TMagic           # magic of operation
    baseTypeMatch: bool      # needed for conversions from T to openarray[T]
                             # for example
    fauxMatch*: TTypeKind    # the match was successful only due to the use
                             # of error or wildcard (unknown) types.
                             # this is used to prevent instantiations.
    genericConverter*: bool  # true if a generic converter needs to
                             # be instantiated
    coerceDistincts*: bool   # this is an explicit coercion that can strip away
                             # a distrinct type
    typedescMatched*: bool
    isNoCall*: bool          # misused for generic type instantiations C[T]
    mutabilityProblem*: uint8 # tyVar mismatch
    inferredTypes: seq[PType] # inferred types during the current signature
                              # matching. they will be reset if the matching
                              # is not successful. may replace the bindings
                              # table in the future.
    diagnostics*: seq[string] # \
                              # when diagnosticsEnabled, the matching process
                              # will collect extra diagnostics that will be
                              # displayed to the user.
                              # triggered when overload resolution fails
                              # or when the explain pragma is used. may be
                              # triggered with an idetools command in the
                              # future.
    inheritancePenalty: int   # to prefer closest father object type
    firstMismatch*: int       # position of the first type mismatch for
                              # better error messages
    diagnosticsEnabled*: bool

  TTypeRelFlag* = enum
    trDontBind
    trNoCovariance

  TTypeRelFlags* = set[TTypeRelFlag]

  TTypeRelation* = enum      # order is important!
    isNone, isConvertible,
    isIntConv,
    isSubtype,
    isSubrange,              # subrange of the wanted type; no type conversion
                             # but apart from that counts as ``isSubtype``
    isBothMetaConvertible    # generic proc parameter was matched against
                             # generic type, e.g., map(mySeq, x=>x+1),
                             # maybe recoverable by rerun if the parameter is
                             # the proc's return value
    isInferred,              # generic proc was matched against a concrete type
    isInferredConvertible,   # same as above, but requiring proc CC conversion
    isGeneric,
    isFromIntLit,            # conversion *from* int literal; proven safe
    isEqual

const
  isNilConversion = isConvertible # maybe 'isIntConv' fits better?

proc markUsed*(conf: ConfigRef; info: TLineInfo, s: PSym; usageSym: var PSym)

template hasFauxMatch*(c: TCandidate): bool = c.fauxMatch != tyNone

proc initCandidateAux(ctx: PContext,
                      c: var TCandidate, callee: PType) {.inline.} =
  c.c = ctx
  c.exactMatches = 0
  c.subtypeMatches = 0
  c.convMatches = 0
  c.intConvMatches = 0
  c.genericMatches = 0
  c.state = csEmpty
  c.callee = callee
  c.call = nil
  c.baseTypeMatch = false
  c.genericConverter = false
  c.inheritancePenalty = 0

proc initCandidate*(ctx: PContext, c: var TCandidate, callee: PType) =
  initCandidateAux(ctx, c, callee)
  c.calleeSym = nil
  initIdTable(c.bindings)

proc put(c: var TCandidate, key, val: PType) {.inline.} =
  idTablePut(c.bindings, key, val.skipIntLit)

proc initCandidate*(ctx: PContext, c: var TCandidate, callee: PSym,
                    binding: PNode, calleeScope = -1,
                    diagnosticsEnabled = false) =
  initCandidateAux(ctx, c, callee.typ)
  c.calleeSym = callee
  if callee.kind in skProcKinds and calleeScope == -1:
    if callee.originatingModule == ctx.module:
      c.calleeScope = 2
      var owner = callee
      while true:
        owner = owner.skipGenericOwner
        if owner.kind == skModule: break
        inc c.calleeScope
    else:
      c.calleeScope = 1
  else:
    c.calleeScope = calleeScope
  c.diagnostics = if diagnosticsEnabled: @[] else: nil
  c.diagnosticsEnabled = diagnosticsEnabled
  c.magic = c.calleeSym.magic
  initIdTable(c.bindings)
  if binding != nil and callee.kind in routineKinds:
    var typeParams = callee.ast[genericParamsPos]
    for i in 1..min(sonsLen(typeParams), sonsLen(binding)-1):
      var formalTypeParam = typeParams.sons[i-1].typ
      var bound = binding[i].typ
      if bound != nil:
        if formalTypeParam.kind == tyTypeDesc:
          if bound.kind != tyTypeDesc:
            bound = makeTypeDesc(ctx, bound)
        else:
          bound = bound.skipTypes({tyTypeDesc})
        put(c, formalTypeParam, bound)

proc newCandidate*(ctx: PContext, callee: PSym,
                   binding: PNode, calleeScope = -1): TCandidate =
  initCandidate(ctx, result, callee, binding, calleeScope)

proc newCandidate*(ctx: PContext, callee: PType): TCandidate =
  initCandidate(ctx, result, callee)

proc copyCandidate(a: var TCandidate, b: TCandidate) =
  a.c = b.c
  a.exactMatches = b.exactMatches
  a.subtypeMatches = b.subtypeMatches
  a.convMatches = b.convMatches
  a.intConvMatches = b.intConvMatches
  a.genericMatches = b.genericMatches
  a.state = b.state
  a.callee = b.callee
  a.calleeSym = b.calleeSym
  a.call = copyTree(b.call)
  a.baseTypeMatch = b.baseTypeMatch
  copyIdTable(a.bindings, b.bindings)

proc sumGeneric(t: PType): int =
  var t = t
  var isvar = 1
  while true:
    case t.kind
    of tyGenericInst, tyArray, tyRef, tyPtr, tyDistinct,
        tyOpenArray, tyVarargs, tySet, tyRange, tySequence, tyGenericBody,
        tyLent:
      t = t.lastSon
      inc result
    of tyOr:
      var maxBranch = 0
      for branch in t.sons:
        let branchSum = branch.sumGeneric
        if branchSum > maxBranch: maxBranch = branchSum
      inc result, maxBranch + 1
      break
    of tyVar:
      t = t.sons[0]
      inc result
      inc isvar
    of tyTypeDesc:
      t = t.lastSon
      if t.kind == tyEmpty: break
      inc result
    of tyGenericInvocation, tyTuple, tyProc, tyAnd:
      result += ord(t.kind in {tyGenericInvocation, tyAnd})
      for i in 0 ..< t.len:
        if t.sons[i] != nil:
          result += t.sons[i].sumGeneric
      break
    of tyStatic:
      return t.sons[0].sumGeneric + 1
    of tyGenericParam, tyExpr, tyStmt: break
    of tyAlias, tySink: t = t.lastSon
    of tyBool, tyChar, tyEnum, tyObject, tyPointer,
        tyString, tyCString, tyInt..tyInt64, tyFloat..tyFloat128,
        tyUInt..tyUInt64, tyCompositeTypeClass:
      return isvar
    else:
      return 0

#var ggDebug: bool

proc complexDisambiguation(a, b: PType): int =
  # 'a' matches better if *every* argument matches better or equal than 'b'.
  var winner = 0
  for i in 1 ..< min(a.len, b.len):
    let x = a.sons[i].sumGeneric
    let y = b.sons[i].sumGeneric
    #if ggDebug:
    #echo "came herA ", typeToString(a.sons[i]), " ", x
    #echo "came herB ", typeToString(b.sons[i]), " ", y
    if x != y:
      if winner == 0:
        if x > y: winner = 1
        else: winner = -1
      elif x > y:
        if winner != 1:
          # contradiction
          return 0
      else:
        if winner != -1:
          return 0
  result = winner
  when false:
    var x, y: int
    for i in 1 ..< a.len: x += a.sons[i].sumGeneric
    for i in 1 ..< b.len: y += b.sons[i].sumGeneric
    result = x - y

proc writeMatches*(c: TCandidate) =
  echo "Candidate '", c.calleeSym.name.s, "' at ", c.c.config $ c.calleeSym.info
  echo "  exact matches: ", c.exactMatches
  echo "  generic matches: ", c.genericMatches
  echo "  subtype matches: ", c.subtypeMatches
  echo "  intconv matches: ", c.intConvMatches
  echo "  conv matches: ", c.convMatches
  echo "  inheritance: ", c.inheritancePenalty

proc cmpCandidates*(a, b: TCandidate): int =
  result = a.exactMatches - b.exactMatches
  if result != 0: return
  result = a.genericMatches - b.genericMatches
  if result != 0: return
  result = a.subtypeMatches - b.subtypeMatches
  if result != 0: return
  result = a.intConvMatches - b.intConvMatches
  if result != 0: return
  result = a.convMatches - b.convMatches
  if result != 0: return
  # the other way round because of other semantics:
  result = b.inheritancePenalty - a.inheritancePenalty
  if result != 0: return
  # prefer more specialized generic over more general generic:
  result = complexDisambiguation(a.callee, b.callee)
  # only as a last resort, consider scoping:
  if result != 0: return
  result = a.calleeScope - b.calleeScope

proc argTypeToString(arg: PNode; prefer: TPreferedDesc): string =
  if arg.kind in nkSymChoices:
    result = typeToString(arg[0].typ, prefer)
    for i in 1 ..< arg.len:
      result.add(" | ")
      result.add typeToString(arg[i].typ, prefer)
  elif arg.typ == nil:
    result = "void"
  else:
    result = arg.typ.typeToString(prefer)

proc describeArgs*(c: PContext, n: PNode, startIdx = 1;
                   prefer: TPreferedDesc = preferName): string =
  result = ""
  for i in countup(startIdx, n.len - 1):
    var arg = n.sons[i]
    if n.sons[i].kind == nkExprEqExpr:
      add(result, renderTree(n.sons[i].sons[0]))
      add(result, ": ")
      if arg.typ.isNil and arg.kind notin {nkStmtList, nkDo}:
        # XXX we really need to 'tryExpr' here!
        arg = c.semOperand(c, n.sons[i].sons[1])
        n.sons[i].typ = arg.typ
        n.sons[i].sons[1] = arg
    else:
      if arg.typ.isNil and arg.kind notin {nkStmtList, nkDo, nkElse,
                                           nkOfBranch, nkElifBranch,
                                           nkExceptBranch}:
        arg = c.semOperand(c, n.sons[i])
        n.sons[i] = arg
    if arg.typ != nil and arg.typ.kind == tyError: return
    add(result, argTypeToString(arg, prefer))
    if i != sonsLen(n) - 1: add(result, ", ")

proc typeRelImpl*(c: var TCandidate, f, aOrig: PType,
                  flags: TTypeRelFlags = {}): TTypeRelation

const traceTypeRel = false

when traceTypeRel:
  var nextTypeRel = 0

template typeRel*(c: var TCandidate, f, aOrig: PType,
                 flags: TTypeRelFlags = {}): TTypeRelation =
  when traceTypeRel:
    var enteringAt = nextTypeRel
    if mdbg:
      inc nextTypeRel
      echo "----- TYPE REL ", enteringAt
      debug f
      debug aOrig
      # writeStackTrace()

  let r = typeRelImpl(c, f, aOrig, flags)

  when traceTypeRel:
    if enteringAt != nextTypeRel:
      echo "----- TYPE REL ", enteringAt, " RESULT: ", r

  r

proc concreteType(c: TCandidate, t: PType): PType =
  case t.kind
  of tyNil:
    result = nil              # what should it be?
  of tyTypeDesc:
    if c.isNoCall: result = t
    else: result = nil
  of tySequence, tySet:
    if t.sons[0].kind == tyEmpty: result = nil
    else: result = t
  of tyGenericParam, tyAnything:
    result = t
    while true:
      result = PType(idTableGet(c.bindings, t))
      if result == nil:
        break # it's ok, no match
        # example code that triggers it:
        # proc sort[T](cmp: proc(a, b: T): int = cmp)
      if result.kind != tyGenericParam: break
  of tyGenericInvocation:
    result = t
    doAssert(false, "cannot resolve type: " & typeToString(t))
  else:
    result = t                # Note: empty is valid here

proc handleRange(f, a: PType, min, max: TTypeKind): TTypeRelation =
  if a.kind == f.kind:
    result = isEqual
  else:
    let ab = skipTypes(a, {tyRange})
    let k = ab.kind
    if k == f.kind: result = isSubrange
    elif k == tyInt and f.kind in {tyRange, tyInt8..tyInt64,
                                   tyUInt..tyUInt64} and
        isIntLit(ab) and ab.n.intVal >= firstOrd(nil, f) and
                         ab.n.intVal <= lastOrd(nil, f):
      # passing 'nil' to firstOrd/lastOrd here as type checking rules should
      # not depent on the target integer size configurations!
      # integer literal in the proper range; we want ``i16 + 4`` to stay an
      # ``int16`` operation so we declare the ``4`` pseudo-equal to int16
      result = isFromIntLit
    elif f.kind == tyInt and k in {tyInt8..tyInt32}:
      result = isIntConv
    elif k >= min and k <= max:
      result = isConvertible
    elif a.kind == tyRange and a.sons[0].kind in {tyInt..tyInt64,
                                                  tyUInt8..tyUInt32} and
                         a.n[0].intVal >= firstOrd(nil, f) and
                         a.n[1].intVal <= lastOrd(nil, f):
      # passing 'nil' to firstOrd/lastOrd here as type checking rules should
      # not depent on the target integer size configurations!
      result = isConvertible
    else: result = isNone
    #elif f.kind == tyInt and k in {tyInt..tyInt32}: result = isIntConv
    #elif f.kind == tyUInt and k in {tyUInt..tyUInt32}: result = isIntConv

proc isConvertibleToRange(f, a: PType): bool =
  # be less picky for tyRange, as that it is used for array indexing:
  if f.kind in {tyInt..tyInt64, tyUInt..tyUInt64} and
     a.kind in {tyInt..tyInt64, tyUInt..tyUInt64}:
    case f.kind
    of tyInt, tyInt64: result = true
    of tyInt8: result = a.kind in {tyInt8, tyInt}
    of tyInt16: result = a.kind in {tyInt8, tyInt16, tyInt}
    of tyInt32: result = a.kind in {tyInt8, tyInt16, tyInt32, tyInt}
    of tyUInt, tyUInt64: result = true
    of tyUInt8: result = a.kind in {tyUInt8, tyUInt}
    of tyUInt16: result = a.kind in {tyUInt8, tyUInt16, tyUInt}
    of tyUInt32: result = a.kind in {tyUInt8, tyUInt16, tyUInt32, tyUInt}
    else: result = false
  elif f.kind in {tyFloat..tyFloat128} and
       a.kind in {tyFloat..tyFloat128}:
    result = true

proc handleFloatRange(f, a: PType): TTypeRelation =
  if a.kind == f.kind:
    result = isEqual
  else:
    let ab = skipTypes(a, {tyRange})
    var k = ab.kind
    if k == f.kind: result = isSubrange
    elif isFloatLit(ab): result = isFromIntLit
    elif isIntLit(ab): result = isConvertible
    elif k >= tyFloat and k <= tyFloat128:
      # conversion to "float32" is not as good:
      if f.kind == tyFloat32: result = isConvertible
      else: result = isIntConv
    else: result = isNone

proc genericParamPut(c: var TCandidate; last, fGenericOrigin: PType) =
 if fGenericOrigin != nil and last.kind == tyGenericInst and
     last.len-1 == fGenericOrigin.len:
   for i in countup(1, sonsLen(fGenericOrigin) - 1):
     let x = PType(idTableGet(c.bindings, fGenericOrigin.sons[i]))
     if x == nil:
       put(c, fGenericOrigin.sons[i], last.sons[i])

proc isObjectSubtype(c: var TCandidate; a, f, fGenericOrigin: PType): int =
  var t = a
  assert t.kind == tyObject
  var depth = 0
  var last = a
  while t != nil and not sameObjectTypes(f, t):
    assert t.kind == tyObject
    t = t.sons[0]
    if t == nil: break
    last = t
    t = skipTypes(t, skipPtrs)
    inc depth
  if t != nil:
    genericParamPut(c, last, fGenericOrigin)
    result = depth
  else:
    result = -1

type
  SkippedPtr = enum skippedNone, skippedRef, skippedPtr

proc skipToObject(t: PType; skipped: var SkippedPtr): PType =
  var r = t
  # we're allowed to skip one level of ptr/ref:
  var ptrs = 0
  while r != nil:
    case r.kind
    of tyGenericInvocation:
      r = r.sons[0]
    of tyRef:
      inc ptrs
      skipped = skippedRef
      r = r.lastSon
    of tyPtr:
      inc ptrs
      skipped = skippedPtr
      r = r.lastSon
    of tyGenericBody, tyGenericInst, tyAlias, tySink:
      r = r.lastSon
    else:
      break
  if r.kind == tyObject and ptrs <= 1: result = r

proc isGenericSubtype(c: var TCandidate; a, f: PType, d: var int, fGenericOrigin: PType = nil): bool =
  assert f.kind in {tyGenericInst, tyGenericInvocation, tyGenericBody}
  var askip = skippedNone
  var fskip = skippedNone
  var t = a.skipToObject(askip)
  let r = f.skipToObject(fskip)
  if r == nil: return false
  var depth = 0
  var last = a
  # XXX sameObjectType can return false here. Need to investigate
  # why that is but sameObjectType does way too much work here anyway.
  while t != nil and r.sym != t.sym and askip == fskip:
    t = t.sons[0]
    if t == nil: break
    last = t
    t = t.skipToObject(askip)
    inc depth
  if t != nil and askip == fskip:
    genericParamPut(c, last, fGenericOrigin)
    d = depth
    result = true

proc minRel(a, b: TTypeRelation): TTypeRelation =
  if a <= b: result = a
  else: result = b

proc recordRel(c: var TCandidate, f, a: PType): TTypeRelation =
  result = isNone
  if sameType(f, a):
    result = isEqual
  elif sonsLen(a) == sonsLen(f):
    result = isEqual
    let firstField = if f.kind == tyTuple: 0
                     else: 1
    for i in countup(firstField, sonsLen(f) - 1):
      var m = typeRel(c, f.sons[i], a.sons[i])
      if m < isSubtype: return isNone
      result = minRel(result, m)
    if f.n != nil and a.n != nil:
      for i in countup(0, sonsLen(f.n) - 1):
        # check field names:
        if f.n.sons[i].kind != nkSym: return isNone
        elif a.n.sons[i].kind != nkSym: return isNone
        else:
          var x = f.n.sons[i].sym
          var y = a.n.sons[i].sym
          if f.kind == tyObject and typeRel(c, x.typ, y.typ) < isSubtype:
            return isNone
          if x.name.id != y.name.id: return isNone

proc allowsNil(f: PType): TTypeRelation {.inline.} =
  result = if tfNotNil notin f.flags: isSubtype else: isNone

proc inconsistentVarTypes(f, a: PType): bool {.inline.} =
  result = f.kind != a.kind and (f.kind in {tyVar, tyLent} or a.kind in {tyVar, tyLent})

proc procParamTypeRel(c: var TCandidate, f, a: PType): TTypeRelation =
  ## For example we have:
  ## .. code-block:: nim
  ##   proc myMap[T,S](sIn: seq[T], f: proc(x: T): S): seq[S] = ...
  ##   proc innerProc[Q,W](q: Q): W = ...
  ## And we want to match: myMap(@[1,2,3], innerProc)
  ## This proc (procParamTypeRel) will do the following steps in
  ## three different calls:
  ## - matches f=T to a=Q. Since f is metatype, we resolve it
  ##    to int (which is already known at this point). So in this case
  ##    Q=int mapping will be saved to c.bindings.
  ## - matches f=S to a=W. Both of these metatypes are unknown, so we
  ##    return with isBothMetaConvertible to ask for rerun.
  ## - matches f=S to a=W. At this point the return type of innerProc
  ##    is known (we get it from c.bindings). We can use that value
  ##    to match with f, and save back to c.bindings.
  var
    f = f
    a = a

  if a.isMetaType:
    let aResolved = PType(idTableGet(c.bindings, a))
    if aResolved != nil:
      a = aResolved
  if a.isMetaType:
    if f.isMetaType:
      # We are matching a generic proc (as proc param)
      # to another generic type appearing in the proc
      # signature. There is a change that the target
      # type is already fully-determined, so we are
      # going to try resolve it
      f = generateTypeInstance(c.c, c.bindings, c.call.info, f)
      if f == nil or f.isMetaType:
        # no luck resolving the type, so the inference fails
        return isBothMetaConvertible
    # Note that this typeRel call will save a's resolved type into c.bindings
    let reverseRel = typeRel(c, a, f)
    if reverseRel >= isGeneric:
      result = isInferred
      #inc c.genericMatches
  else:
    # Note that this typeRel call will save f's resolved type into c.bindings
    # if f is metatype.
    result = typeRel(c, f, a)

  if result <= isSubtype or inconsistentVarTypes(f, a):
    result = isNone

  #if result == isEqual:
  #  inc c.exactMatches

proc procTypeRel(c: var TCandidate, f, a: PType): TTypeRelation =
  case a.kind
  of tyProc:
    if sonsLen(f) != sonsLen(a): return
    result = isEqual      # start with maximum; also correct for no
                          # params at all

    template checkParam(f, a) =
      result = minRel(result, procParamTypeRel(c, f, a))
      if result == isNone: return

    # Note: We have to do unification for the parameters before the
    # return type!
    for i in 1 ..< f.sonsLen:
      checkParam(f.sons[i], a.sons[i])

    if f.sons[0] != nil:
      if a.sons[0] != nil:
        checkParam(f.sons[0], a.sons[0])
      else:
        return isNone
    elif a.sons[0] != nil:
      return isNone

    if tfNoSideEffect in f.flags and tfNoSideEffect notin a.flags:
      return isNone
    elif tfThread in f.flags and a.flags * {tfThread, tfNoSideEffect} == {} and
        optThreadAnalysis in c.c.config.globalOptions:
      # noSideEffect implies ``tfThread``!
      return isNone
    elif f.flags * {tfIterator} != a.flags * {tfIterator}:
      return isNone
    elif f.callConv != a.callConv:
      # valid to pass a 'nimcall' thingie to 'closure':
      if f.callConv == ccClosure and a.callConv == ccDefault:
        result = if result == isInferred: isInferredConvertible
                 elif result == isBothMetaConvertible: isBothMetaConvertible
                 else: isConvertible
      else:
        return isNone
    when useEffectSystem:
      if compatibleEffects(f, a) != efCompat: return isNone

  of tyNil:
    result = f.allowsNil
  else: discard

proc typeRangeRel(f, a: PType): TTypeRelation {.noinline.} =
  let
    a0 = firstOrd(nil, a)
    a1 = lastOrd(nil, a)
    f0 = firstOrd(nil, f)
    f1 = lastOrd(nil, f)
  if a0 == f0 and a1 == f1:
    result = isEqual
  elif a0 >= f0 and a1 <= f1:
    result = isConvertible
  elif a0 <= f1 and f0 <= a1:
    # X..Y and C..D overlap iff (X <= D and C <= Y)
    result = isConvertible
  else:
    result = isNone

proc matchUserTypeClass*(m: var TCandidate; ff, a: PType): PType =
  var
    c = m.c
    typeClass = ff.skipTypes({tyUserTypeClassInst})
    body = typeClass.n[3]
    matchedConceptContext: TMatchedConcept
    prevMatchedConcept = c.matchedConcept
    prevCandidateType = typeClass[0][0]

  if prevMatchedConcept != nil:
    matchedConceptContext.prev = prevMatchedConcept
    matchedConceptContext.depth = prevMatchedConcept.depth + 1
    if prevMatchedConcept.depth > 4:
      localError(m.c.graph.config, body.info, $body & " too nested for type matching")
      return nil

  openScope(c)
  matchedConceptContext.candidateType = a
  typeClass[0].sons[0] = a
  c.matchedConcept = addr(matchedConceptContext)
  defer:
    c.matchedConcept = prevMatchedConcept
    typeClass[0].sons[0] = prevCandidateType
    closeScope(c)

  var typeParams: seq[(PSym, PType)]

  if ff.kind == tyUserTypeClassInst:
    for i in 1 ..< (ff.len - 1):
      var
        typeParamName = ff.base.sons[i-1].sym.name
        typ = ff.sons[i]
        param: PSym
        alreadyBound = PType(idTableGet(m.bindings, typ))

      if alreadyBound != nil: typ = alreadyBound

      template paramSym(kind): untyped =
        newSym(kind, typeParamName, typeClass.sym, typeClass.sym.info, {})

      block addTypeParam:
        for prev in typeParams:
          if prev[1].id == typ.id:
            param = paramSym prev[0].kind
            param.typ = prev[0].typ
            break addTypeParam

        case typ.kind
        of tyStatic:
          param = paramSym skConst
          param.typ = typ.exactReplica
          if typ.n == nil:
            param.typ.flags.incl tfInferrableStatic
          else:
            param.ast = typ.n
        of tyUnknown:
          param = paramSym skVar
          param.typ = typ.exactReplica
        else:
          param = paramSym skType
          param.typ = if typ.isMetaType:
                        c.newTypeWithSons(tyInferred, @[typ])
                      else:
                        makeTypeDesc(c, typ)

        typeParams.safeAdd((param, typ))

      addDecl(c, param)

  var
    oldWriteHook: type(m.c.config.writelnHook)
    diagnostics: seq[string]
    errorPrefix: string
    flags: TExprFlags = {}
    collectDiagnostics = m.diagnosticsEnabled or
                         sfExplain in typeClass.sym.flags

  if collectDiagnostics:
    oldWriteHook = m.c.config.writelnHook
    # XXX: we can't write to m.diagnostics directly, because
    # Nim doesn't support capturing var params in closures
    diagnostics = @[]
    flags = {efExplain}
    m.c.config.writelnHook = proc (s: string) =
      if errorPrefix == nil: errorPrefix = typeClass.sym.name.s & ":"
      let msg = s.replace("Error:", errorPrefix)
      if oldWriteHook != nil: oldWriteHook msg
      diagnostics.add msg

  var checkedBody = c.semTryExpr(c, body.copyTree, flags)

  if collectDiagnostics:
    m.c.config.writelnHook = oldWriteHook
    for msg in diagnostics:
      m.diagnostics.safeAdd msg
      m.diagnosticsEnabled = true

  if checkedBody == nil: return nil

  # The inferrable type params have been identified during the semTryExpr above.
  # We need to put them in the current sigmatch's binding table in order for them
  # to be resolvable while matching the rest of the parameters
  for p in typeParams:
    put(m, p[1], p[0].typ)

  if ff.kind == tyUserTypeClassInst:
    result = generateTypeInstance(c, m.bindings, typeClass.sym.info, ff)
  else:
    result = copyType(ff, ff.owner, true)

  result.n = checkedBody

proc shouldSkipDistinct(m: TCandidate; rules: PNode, callIdent: PIdent): bool =
  # XXX This is bad as 'considerQuotedIdent' can produce an error!
  if rules.kind == nkWith:
    for r in rules:
      if considerQuotedIdent(m.c, r) == callIdent: return true
    return false
  else:
    for r in rules:
      if considerQuotedIdent(m.c, r) == callIdent: return false
    return true

proc maybeSkipDistinct(m: TCandidate; t: PType, callee: PSym): PType =
  if t != nil and t.kind == tyDistinct and t.n != nil and
     shouldSkipDistinct(m, t.n, callee.name):
    result = t.base
  else:
    result = t

proc tryResolvingStaticExpr(c: var TCandidate, n: PNode,
                            allowUnresolved = false): PNode =
  # Consider this example:
  #   type Value[N: static[int]] = object
  #   proc foo[N](a: Value[N], r: range[0..(N-1)])
  # Here, N-1 will be initially nkStaticExpr that can be evaluated only after
  # N is bound to a concrete value during the matching of the first param.
  # This proc is used to evaluate such static expressions.
  let instantiated = replaceTypesInBody(c.c, c.bindings, n, nil,
                                        allowMetaTypes = allowUnresolved)
  result = c.c.semExpr(c.c, instantiated)

proc inferStaticParam*(c: var TCandidate, lhs: PNode, rhs: BiggestInt): bool =
  # This is a simple integer arithimetic equation solver,
  # capable of deriving the value of a static parameter in
  # expressions such as (N + 5) / 2 = rhs
  #
  # Preconditions:
  #
  #   * The input of this proc must be semantized
  #     - all templates should be expanded
  #     - aby constant folding possible should already be performed
  #
  #   * There must be exactly one unresolved static parameter
  #
  # Result:
  #
  #   The proc will return true if the static types was successfully
  #   inferred. The result will be bound to the original static type
  #   in the TCandidate.
  #
  if lhs.kind in nkCallKinds and lhs[0].kind == nkSym:
    case lhs[0].sym.magic
    of mUnaryLt:
      return inferStaticParam(c, lhs[1], rhs + 1)

    of mAddI, mAddU, mInc, mSucc:
      if lhs[1].kind == nkIntLit:
        return inferStaticParam(c, lhs[2], rhs - lhs[1].intVal)
      elif lhs[2].kind == nkIntLit:
        return inferStaticParam(c, lhs[1], rhs - lhs[2].intVal)

    of mDec, mSubI, mSubU, mPred:
      if lhs[1].kind == nkIntLit:
        return inferStaticParam(c, lhs[2], lhs[1].intVal - rhs)
      elif lhs[2].kind == nkIntLit:
        return inferStaticParam(c, lhs[1], rhs + lhs[2].intVal)

    of mMulI, mMulU:
      if lhs[1].kind == nkIntLit:
        if rhs mod lhs[1].intVal == 0:
          return inferStaticParam(c, lhs[2], rhs div lhs[1].intVal)
      elif lhs[2].kind == nkIntLit:
        if rhs mod lhs[2].intVal == 0:
          return inferStaticParam(c, lhs[1], rhs div lhs[2].intVal)

    of mDivI, mDivU:
      if lhs[1].kind == nkIntLit:
        if lhs[1].intVal mod rhs == 0:
          return inferStaticParam(c, lhs[2], lhs[1].intVal div rhs)
      elif lhs[2].kind == nkIntLit:
        return inferStaticParam(c, lhs[1], lhs[2].intVal * rhs)

    of mShlI:
      if lhs[2].kind == nkIntLit:
        return inferStaticParam(c, lhs[1], rhs shr lhs[2].intVal)

    of mShrI:
      if lhs[2].kind == nkIntLit:
        return inferStaticParam(c, lhs[1], rhs shl lhs[2].intVal)

    of mUnaryMinusI:
      return inferStaticParam(c, lhs[1], -rhs)

    of mUnaryPlusI, mToInt, mToBiggestInt:
      return inferStaticParam(c, lhs[1], rhs)

    else: discard

  elif lhs.kind == nkSym and lhs.typ.kind == tyStatic and lhs.typ.n == nil:
    var inferred = newTypeWithSons(c.c, tyStatic, lhs.typ.sons)
    inferred.n = newIntNode(nkIntLit, rhs)
    put(c, lhs.typ, inferred)
    if c.c.matchedConcept != nil:
      # inside concepts, binding is currently done with
      # direct mutation of the involved types:
      lhs.typ.n = inferred.n
    return true

  return false

proc failureToInferStaticParam(conf: ConfigRef; n: PNode) =
  let staticParam = n.findUnresolvedStatic
  let name = if staticParam != nil: staticParam.sym.name.s
             else: "unknown"
  localError(conf, n.info, "cannot infer the value of the static param '" & name & "'")

proc inferStaticsInRange(c: var TCandidate,
                         inferred, concrete: PType): TTypeRelation =
  let lowerBound = tryResolvingStaticExpr(c, inferred.n[0],
                                          allowUnresolved = true)
  let upperBound = tryResolvingStaticExpr(c, inferred.n[1],
                                          allowUnresolved = true)
  template doInferStatic(e: PNode, r: BiggestInt) =
    var exp = e
    var rhs = r
    if inferStaticParam(c, exp, rhs):
      return isGeneric
    else:
      failureToInferStaticParam(c.c.config, exp)

  if lowerBound.kind == nkIntLit:
    if upperBound.kind == nkIntLit:
      if lengthOrd(c.c.config, concrete) == upperBound.intVal - lowerBound.intVal + 1:
        return isGeneric
      else:
        return isNone
    doInferStatic(upperBound, lengthOrd(c.c.config, concrete) + lowerBound.intVal - 1)
  elif upperBound.kind == nkIntLit:
    doInferStatic(lowerBound, upperBound.intVal + 1 - lengthOrd(c.c.config, concrete))

template subtypeCheck() =
  if result <= isSubrange and f.lastSon.skipTypes(abstractInst).kind in {tyRef, tyPtr, tyVar, tyLent}:
    result = isNone

proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
  # this proc is always called for a pair of matching types
  assert f.kind == a.kind

  template baseTypesCheck(lhs, rhs: PType): bool =
    lhs.kind notin {tyPtr, tyRef, tyVar, tyLent} and
      typeRel(c, lhs, rhs, {trNoCovariance}) == isSubtype

  case f.kind
  of tyRef, tyPtr:
    return baseTypesCheck(f.base, a.base)
  of tyGenericInst:
    let body = f.base
    return body == a.base and
           a.sonsLen == 3 and
           tfWeakCovariant notin body.sons[0].flags and
           baseTypesCheck(f.sons[1], a.sons[1])
  else:
    return false

when false:
  proc maxNumericType(prev, candidate: PType): PType =
    let c = candidate.skipTypes({tyRange})
    template greater(s) =
      if c.kind in s: result = c
    case prev.kind
    of tyInt: greater({tyInt64})
    of tyInt8: greater({tyInt, tyInt16, tyInt32, tyInt64})
    of tyInt16: greater({tyInt, tyInt32, tyInt64})
    of tyInt32: greater({tyInt64})

    of tyUInt: greater({tyUInt64})
    of tyUInt8: greater({tyUInt, tyUInt16, tyUInt32, tyUInt64})
    of tyUInt16: greater({tyUInt, tyUInt32, tyUInt64})
    of tyUInt32: greater({tyUInt64})

    of tyFloat32: greater({tyFloat64, tyFloat128})
    of tyFloat64: greater({tyFloat128})
    else: discard

proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
                 flags: TTypeRelFlags = {}): TTypeRelation =
  # typeRel can be used to establish various relationships between types:
  #
  # 1) When used with concrete types, it will check for type equivalence
  # or a subtype relationship.
  #
  # 2) When used with a concrete type against a type class (such as generic
  # signature of a proc), it will check whether the concrete type is a member
  # of the designated type class.
  #
  # 3) When used with two type classes, it will check whether the types
  # matching the first type class are a strict subset of the types matching
  # the other. This allows us to compare the signatures of generic procs in
  # order to give preferrence to the most specific one:
  #
  # seq[seq[any]] is a strict subset of seq[any] and hence more specific.

  result = isNone
  assert(f != nil)

  if f.kind == tyExpr:
    if aOrig != nil: put(c, f, aOrig)
    return isGeneric

  assert(aOrig != nil)

  var
    useTypeLoweringRuleInTypeClass = c.c.matchedConcept != nil and
                                     not c.isNoCall and
                                     f.kind != tyTypeDesc and
                                     tfExplicit notin aOrig.flags

    aOrig = if useTypeLoweringRuleInTypeClass:
          aOrig.skipTypes({tyTypeDesc})
        else:
          aOrig

  if aOrig.kind == tyInferred:
    let prev = aOrig.previouslyInferred
    if prev != nil:
      return typeRel(c, f, prev)
    else:
      var candidate = f

      case f.kind
      of tyGenericParam:
        var prev  = PType(idTableGet(c.bindings, f))
        if prev != nil: candidate = prev
      of tyFromExpr:
        let computedType = tryResolvingStaticExpr(c, f.n).typ
        case computedType.kind
        of tyTypeDesc:
          candidate = computedType.base
        of tyStatic:
          candidate = computedType
        else:
          # XXX What is this non-sense? Error reporting in signature matching?
          discard "localError(f.n.info, errTypeExpected)"
      else:
        discard

      result = typeRel(c, aOrig.base, candidate)
      if result != isNone:
        c.inferredTypes.safeAdd aOrig
        aOrig.sons.add candidate
        result = isEqual
      return

  template doBind: bool = trDontBind notin flags

  # var and static arguments match regular modifier-free types
  var a = maybeSkipDistinct(c, aOrig.skipTypes({tyStatic, tyVar, tyLent}), c.calleeSym)
  # XXX: Theoretically, maybeSkipDistinct could be called before we even
  # start the param matching process. This could be done in `prepareOperand`
  # for example, but unfortunately `prepareOperand` is not called in certain
  # situation when nkDotExpr are rotated to nkDotCalls

  if aOrig.kind in {tyAlias, tySink}:
    return typeRel(c, f, lastSon(aOrig))

  if a.kind == tyGenericInst and
      skipTypes(f, {tyVar, tyLent}).kind notin {
        tyGenericBody, tyGenericInvocation,
        tyGenericInst, tyGenericParam} + tyTypeClasses:
    return typeRel(c, f, lastSon(a))

  if a.isResolvedUserTypeClass:
    return typeRel(c, f, a.lastSon)

  template bindingRet(res) =
    if doBind:
      let bound = aOrig.skipTypes({tyRange}).skipIntLit
      put(c, f, bound)
    return res

  template considerPreviousT(body: untyped) =
    var prev = PType(idTableGet(c.bindings, f))
    if prev == nil: body
    else: return typeRel(c, prev, a)

  case a.kind
  of tyOr:
    # XXX: deal with the current dual meaning of tyGenericParam
    c.typedescMatched = true
    # seq[int|string] vs seq[number]
    # both int and string must match against number
    # but ensure that '[T: A|A]' matches as good as '[T: A]' (bug #2219):
    result = isGeneric
    for branch in a.sons:
      let x = typeRel(c, f, branch, flags + {trDontBind})
      if x == isNone: return isNone
      if x < result: result = x
    return

  of tyAnd:
    # XXX: deal with the current dual meaning of tyGenericParam
    c.typedescMatched = true
    # seq[Sortable and Iterable] vs seq[Sortable]
    # only one match is enough
    for branch in a.sons:
      let x = typeRel(c, f, branch, flags + {trDontBind})
      if x != isNone:
        return if x >= isGeneric: isGeneric else: x
    return isNone

  of tyNot:
    case f.kind
    of tyNot:
      # seq[!int] vs seq[!number]
      # seq[float] matches the first, but not the second
      # we must turn the problem around:
      # is number a subset of int?
      return typeRel(c, a.lastSon, f.lastSon)

    else:
      # negative type classes are essentially infinite,
      # so only the `any` type class is their superset
      return if f.kind == tyAnything: isGeneric
             else: isNone

  of tyAnything:
    return if f.kind == tyAnything: isGeneric
           else: isNone

  of tyUserTypeClass, tyUserTypeClassInst:
    if c.c.matchedConcept != nil and c.c.matchedConcept.depth <= 4:
      # consider this: 'var g: Node' *within* a concept where 'Node'
      # is a concept too (tgraph)
      inc c.c.matchedConcept.depth
      let x = typeRel(c, a, f, flags + {trDontBind})
      if x >= isGeneric:
        return isGeneric
  else: discard

  case f.kind
  of tyEnum:
    if a.kind == f.kind and sameEnumTypes(f, a): result = isEqual
    elif sameEnumTypes(f, skipTypes(a, {tyRange})): result = isSubtype
  of tyBool, tyChar:
    if a.kind == f.kind: result = isEqual
    elif skipTypes(a, {tyRange}).kind == f.kind: result = isSubtype
  of tyRange:
    if a.kind == f.kind:
      if f.base.kind == tyNone: return isGeneric
      result = typeRel(c, base(f), base(a))
      # bugfix: accept integer conversions here
      #if result < isGeneric: result = isNone
      if result notin {isNone, isGeneric}:
        # resolve any late-bound static expressions
        # that may appear in the range:
        for i in 0..1:
          if f.n[i].kind == nkStaticExpr:
            f.n.sons[i] = tryResolvingStaticExpr(c, f.n[i])
        result = typeRangeRel(f, a)
    else:
      if skipTypes(f, {tyRange}).kind == a.kind:
        result = isIntConv
      elif isConvertibleToRange(skipTypes(f, {tyRange}), a):
        result = isConvertible  # a convertible to f
  of tyInt:      result = handleRange(f, a, tyInt8, tyInt32)
  of tyInt8:     result = handleRange(f, a, tyInt8, tyInt8)
  of tyInt16:    result = handleRange(f, a, tyInt8, tyInt16)
  of tyInt32:    result = handleRange(f, a, tyInt8, tyInt32)
  of tyInt64:    result = handleRange(f, a, tyInt, tyInt64)
  of tyUInt:     result = handleRange(f, a, tyUInt8, tyUInt32)
  of tyUInt8:    result = handleRange(f, a, tyUInt8, tyUInt8)
  of tyUInt16:   result = handleRange(f, a, tyUInt8, tyUInt16)
  of tyUInt32:   result = handleRange(f, a, tyUInt8, tyUInt32)
  of tyUInt64:   result = handleRange(f, a, tyUInt, tyUInt64)
  of tyFloat:    result = handleFloatRange(f, a)
  of tyFloat32:  result = handleFloatRange(f, a)
  of tyFloat64:  result = handleFloatRange(f, a)
  of tyFloat128: result = handleFloatRange(f, a)
  of tyVar, tyLent:
    if aOrig.kind == f.kind: result = typeRel(c, f.base, aOrig.base)
    else: result = typeRel(c, f.base, aOrig, flags + {trNoCovariance})
    subtypeCheck()
  of tyArray:
    case a.kind
    of tyArray:
      var fRange = f.sons[0]
      var aRange = a.sons[0]
      if fRange.kind == tyGenericParam:
        var prev = PType(idTableGet(c.bindings, fRange))
        if prev == nil:
          put(c, fRange, a.sons[0])
          fRange = a
        else:
          fRange = prev
      let ff = f.sons[1].skipTypes({tyTypeDesc})
      # This typeDesc rule is wrong, see bug #7331
      let aa = a.sons[1] #.skipTypes({tyTypeDesc})

      if f.sons[0].kind != tyGenericParam and aa.kind == tyEmpty:
        result = isGeneric
      else:
        result = typeRel(c, ff, aa)

      if result < isGeneric:
        if nimEnableCovariance and
           trNoCovariance notin flags and
           ff.kind == aa.kind and
           isCovariantPtr(c, ff, aa):
          result = isSubtype
        else:
          return isNone

      if fRange.rangeHasUnresolvedStatic:
        return inferStaticsInRange(c, fRange, a)
      elif c.c.matchedConcept != nil and aRange.rangeHasUnresolvedStatic:
        return inferStaticsInRange(c, aRange, f)
      else:
        if lengthOrd(c.c.config, fRange) != lengthOrd(c.c.config, aRange):
          result = isNone
    else: discard
  of tyOpenArray, tyVarargs:
    # varargs[expr] is special too but handled earlier. So we only need to
    # handle varargs[stmt] which is the same as varargs[typed]:
    if f.kind == tyVarargs:
      if tfVarargs in a.flags:
        return typeRel(c, f.base, a.lastSon)
      if tfOldSchoolExprStmt in f.sons[0].flags:
        if f.sons[0].kind == tyExpr: return
      elif f.sons[0].kind == tyStmt: return

    template matchArrayOrSeq(aBase: PType) =
      let ff = f.base
      let aa = aBase
      let baseRel = typeRel(c, ff, aa)
      if baseRel >= isGeneric:
        result = isConvertible
      elif nimEnableCovariance and
           trNoCovariance notin flags and
           ff.kind == aa.kind and
           isCovariantPtr(c, ff, aa):
        result = isConvertible

    case a.kind
    of tyOpenArray, tyVarargs:
      result = typeRel(c, base(f), base(a))
      if result < isGeneric: result = isNone
    of tyArray:
      if (f.sons[0].kind != tyGenericParam) and (a.sons[1].kind == tyEmpty):
        return isSubtype
      matchArrayOrSeq(a.sons[1])
    of tySequence:
      if (f.sons[0].kind != tyGenericParam) and (a.sons[0].kind == tyEmpty):
        return isConvertible
      matchArrayOrSeq(a.sons[0])
    of tyString:
      if f.kind == tyOpenArray:
        if f.sons[0].kind == tyChar:
          result = isConvertible
        elif f.sons[0].kind == tyGenericParam and a.len > 0 and
            typeRel(c, base(f), base(a)) >= isGeneric:
          result = isConvertible
    else: discard
  of tySequence:
    case a.kind
    of tySequence:
      if (f.sons[0].kind != tyGenericParam) and (a.sons[0].kind == tyEmpty):
        result = isSubtype
      else:
        let ff = f.sons[0]
        let aa = a.sons[0]
        result = typeRel(c, ff, aa)
        if result < isGeneric:
          if nimEnableCovariance and
             trNoCovariance notin flags and
             ff.kind == aa.kind and
             isCovariantPtr(c, ff, aa):
            result = isSubtype
          else:
            result = isNone
        elif tfNotNil in f.flags and tfNotNil notin a.flags:
          result = isNilConversion
    of tyNil: result = f.allowsNil
    else: discard
  of tyOrdinal:
    if isOrdinalType(a):
      var x = if a.kind == tyOrdinal: a.sons[0] else: a
      if f.sons[0].kind == tyNone:
        result = isGeneric
      else:
        result = typeRel(c, f.sons[0], x)
        if result < isGeneric: result = isNone
    elif a.kind == tyGenericParam:
      result = isGeneric
  of tyForward:
    #internalError("forward type in typeRel()")
    result = isNone
  of tyNil:
    if a.kind == f.kind: result = isEqual
  of tyTuple:
    if a.kind == tyTuple: result = recordRel(c, f, a)
  of tyObject:
    if a.kind == tyObject:
      if sameObjectTypes(f, a):
        result = isEqual
        # elif tfHasMeta in f.flags: result = recordRel(c, f, a)
      else:
        var depth = isObjectSubtype(c, a, f, nil)
        if depth > 0:
          inc(c.inheritancePenalty, depth)
          result = isSubtype
  of tyDistinct:
    if a.kind == tyDistinct:
      if sameDistinctTypes(f, a): result = isEqual
      elif f.base.kind == tyAnything: result = isGeneric
      elif c.coerceDistincts: result = typeRel(c, f.base, a)
    elif a.kind == tyNil and f.base.kind in NilableTypes:
      result = f.allowsNil
    elif c.coerceDistincts: result = typeRel(c, f.base, a)
  of tySet:
    if a.kind == tySet:
      if f.sons[0].kind != tyGenericParam and a.sons[0].kind == tyEmpty:
        result = isSubtype
      else:
        result = typeRel(c, f.sons[0], a.sons[0])
        if result <= isConvertible:
          result = isNone     # BUGFIX!
  of tyPtr, tyRef:
    if a.kind == f.kind:
      # ptr[R, T] can be passed to ptr[T], but not the other way round:
      if a.len < f.len: return isNone
      for i in 0..f.len-2:
        if typeRel(c, f.sons[i], a.sons[i]) == isNone: return isNone
      result = typeRel(c, f.lastSon, a.lastSon, flags + {trNoCovariance})
      subtypeCheck()
      if result <= isConvertible: result = isNone
      elif tfNotNil in f.flags and tfNotNil notin a.flags:
        result = isNilConversion
    elif a.kind == tyNil: result = f.allowsNil
    else: discard
  of tyProc:
    result = procTypeRel(c, f, a)
    if result != isNone and tfNotNil in f.flags and tfNotNil notin a.flags:
      result = isNilConversion
  of tyPointer:
    case a.kind
    of tyPointer:
      if tfNotNil in f.flags and tfNotNil notin a.flags:
        result = isNilConversion
      else:
        result = isEqual
    of tyNil: result = f.allowsNil
    of tyProc:
      if a.callConv != ccClosure: result = isConvertible
    of tyPtr:
      # 'pointer' is NOT compatible to regionized pointers
      # so 'dealloc(regionPtr)' fails:
      if a.len == 1: result = isConvertible
    of tyCString: result = isConvertible
    else: discard
  of tyString:
    case a.kind
    of tyString:
      if tfNotNil in f.flags and tfNotNil notin a.flags:
        result = isNilConversion
      else:
        result = isEqual
    of tyNil: result = f.allowsNil
    else: discard
  of tyCString:
    # conversion from string to cstring is automatic:
    case a.kind
    of tyCString:
      if tfNotNil in f.flags and tfNotNil notin a.flags:
        result = isNilConversion
      else:
        result = isEqual
    of tyNil: result = f.allowsNil
    of tyString: result = isConvertible
    of tyPtr:
      # ptr[Tag, char] is not convertible to 'cstring' for now:
      if a.len == 1:
        let pointsTo = a.sons[0].skipTypes(abstractInst)
        if pointsTo.kind == tyChar: result = isConvertible
        elif pointsTo.kind == tyArray and firstOrd(nil, pointsTo.sons[0]) == 0 and
            skipTypes(pointsTo.sons[0], {tyRange}).kind in {tyInt..tyInt64} and
            pointsTo.sons[1].kind == tyChar:
          result = isConvertible
    else: discard

  of tyEmpty, tyVoid:
    if a.kind == f.kind: result = isEqual

  of tyAlias, tySink:
    result = typeRel(c, lastSon(f), a)

  of tyGenericInst:
    var prev = PType(idTableGet(c.bindings, f))
    var f = if prev == nil: f else: prev

    let roota = a.skipGenericAlias
    let rootf = f.skipGenericAlias

    var m = c
    if a.kind == tyGenericInst:
      if roota.base == rootf.base:
        let nextFlags = flags + {trNoCovariance}
        var hasCovariance = false
        for i in 1 .. rootf.sonsLen-2:
          let ff = rootf.sons[i]
          let aa = roota.sons[i]
          result = typeRel(c, ff, aa, nextFlags)
          if result notin {isEqual, isGeneric}:
            if trNoCovariance notin flags and ff.kind == aa.kind:
              let paramFlags = rootf.base.sons[i-1].flags
              hasCovariance =
                if tfCovariant in paramFlags:
                  if tfWeakCovariant in paramFlags:
                    isCovariantPtr(c, ff, aa)
                  else:
                    ff.kind notin {tyRef, tyPtr} and result == isSubtype
                else:
                  tfContravariant in paramFlags and
                    typeRel(c, aa, ff) == isSubtype
              if hasCovariance:
                continue

            return isNone
        if prev == nil: put(c, f, a)
        result = isGeneric
      else:
        let fKind = rootf.lastSon.kind
        if fKind in {tyAnd, tyOr}:
          result = typeRel(c, lastSon(f), a)
          if result != isNone: put(c, f, a)
          return

        var aAsObject = roota.lastSon

        if fKind in {tyRef, tyPtr}:
          if aAsObject.kind == tyObject:
            # bug #7600, tyObject cannot be passed
            # as argument to tyRef/tyPtr
            return isNone
          elif aAsObject.kind == fKind:
            aAsObject = aAsObject.base

        if aAsObject.kind == tyObject:
          let baseType = aAsObject.base
          if baseType != nil:
            c.inheritancePenalty += 1
            return typeRel(c, f, baseType)

        result = isNone
    else:
      result = typeRel(c, lastSon(f), a)
      if result != isNone and a.kind != tyNil:
        put(c, f, a)

  of tyGenericBody:
    considerPreviousT:
      if a.kind == tyGenericInst and a.sons[0] == f:
        bindingRet isGeneric
      let ff = lastSon(f)
      if ff != nil:
        result = typeRel(c, ff, a)

  of tyGenericInvocation:
    var x = a.skipGenericAlias
    # XXX: This is very hacky. It should be moved back into liftTypeParam
    if x.kind in {tyGenericInst, tyArray} and
       c.calleeSym != nil and
       c.calleeSym.kind in {skProc, skFunc} and c.call != nil:
      let inst = prepareMetatypeForSigmatch(c.c, c.bindings, c.call.info, f)
      return typeRel(c, inst, a)

    var depth = 0
    if x.kind == tyGenericInvocation or f.sons[0].kind != tyGenericBody:
      #InternalError("typeRel: tyGenericInvocation -> tyGenericInvocation")
      # simply no match for now:
      discard
    elif x.kind == tyGenericInst and
          ((f.sons[0] == x.sons[0]) or isGenericSubType(c, x, f, depth)) and
          (sonsLen(x) - 1 == sonsLen(f)):
      for i in countup(1, sonsLen(f) - 1):
        if x.sons[i].kind == tyGenericParam:
          internalError(c.c.graph.config, "wrong instantiated type!")
        elif typeRel(c, f.sons[i], x.sons[i]) <= isSubtype:
          # Workaround for regression #4589
          if f.sons[i].kind != tyTypeDesc: return
      c.inheritancePenalty += depth
      result = isGeneric
    else:
      let genericBody = f.sons[0]
      var askip = skippedNone
      var fskip = skippedNone
      let aobj = x.skipToObject(askip)
      let fobj = genericBody.lastSon.skipToObject(fskip)
      var depth = -1
      if fobj != nil and aobj != nil and askip == fskip:
        depth = isObjectSubtype(c, aobj, fobj, f)
      result = typeRel(c, genericBody, x)
      if result != isNone:
        # see tests/generics/tgeneric3.nim for an example that triggers this
        # piece of code:
        #
        # proc internalFind[T,D](n: PNode[T,D], key: T): ref TItem[T,D]
        # proc internalPut[T,D](ANode: ref TNode[T,D], Akey: T, Avalue: D,
        #                       Oldvalue: var D): ref TNode[T,D]
        # var root = internalPut[int, int](nil, 312, 312, oldvalue)
        # var it1 = internalFind(root, 312) # cannot instantiate: 'D'
        #
        # we steal the generic parameters from the tyGenericBody:
        for i in countup(1, sonsLen(f) - 1):
          let x = PType(idTableGet(c.bindings, genericBody.sons[i-1]))
          if x == nil:
            discard "maybe fine (for eg. a==tyNil)"
          elif x.kind in {tyGenericInvocation, tyGenericParam}:
            internalError(c.c.graph.config, "wrong instantiated type!")
          else:
            put(c, f.sons[i], x)

      if result == isNone:
        # Here object inheriting from generic/specialized generic object
        # crossing path with metatypes/aliases, so we need to separate them
        # by checking sym.id
        let genericSubtype = isGenericSubType(c, x, f, depth, f)
        if not (genericSubtype and aobj.sym.id != fobj.sym.id) and aOrig.kind != tyGenericBody:
          depth = -1

      if depth >= 0:
        c.inheritancePenalty += depth
        # bug #4863: We still need to bind generic alias crap, so
        # we cannot return immediately:
        result = if depth == 0: isGeneric else: isSubtype
  of tyAnd:
    considerPreviousT:
      result = isEqual
      for branch in f.sons:
        let x = typeRel(c, branch, aOrig)
        if x < isSubtype: return isNone
        # 'and' implies minimum matching result:
        if x < result: result = x
      if result > isGeneric: result = isGeneric
      bindingRet result

  of tyOr:
    considerPreviousT:
      result = isNone
      let oldInheritancePenalty = c.inheritancePenalty
      var maxInheritance = 0
      for branch in f.sons:
        c.inheritancePenalty = 0
        let x = typeRel(c, branch, aOrig)
        maxInheritance = max(maxInheritance, c.inheritancePenalty)

        # 'or' implies maximum matching result:
        if x > result: result = x
      if result >= isSubtype:
        if result > isGeneric: result = isGeneric
        bindingRet result
      else:
        result = isNone
      c.inheritancePenalty = oldInheritancePenalty + maxInheritance

  of tyNot:
    considerPreviousT:
      for branch in f.sons:
        if typeRel(c, branch, aOrig) != isNone:
          return isNone

      bindingRet isGeneric

  of tyAnything:
    considerPreviousT:
      var concrete = concreteType(c, a)
      if concrete != nil and doBind:
        put(c, f, concrete)
      return isGeneric

  of tyBuiltInTypeClass:
    considerPreviousT:
      let targetKind = f.sons[0].kind
      let effectiveArgType = a.skipTypes({tyRange, tyGenericInst,
                                          tyBuiltInTypeClass, tyAlias, tySink})
      let typeClassMatches = targetKind == effectiveArgType.kind and
                             not effectiveArgType.isEmptyContainer
      if typeClassMatches or
        (targetKind in {tyProc, tyPointer} and effectiveArgType.kind == tyNil):
        put(c, f, a)
        return isGeneric
      else:
        return isNone

  of tyUserTypeClassInst, tyUserTypeClass:
    if f.isResolvedUserTypeClass:
      result = typeRel(c, f.lastSon, a)
    else:
      considerPreviousT:
        if aOrig == f: return isEqual
        var matched = matchUserTypeClass(c, f, aOrig)
        if matched != nil:
          bindConcreteTypeToUserTypeClass(matched, a)
          if doBind: put(c, f, matched)
          result = isGeneric
        else:
          result = isNone

  of tyCompositeTypeClass:
    considerPreviousT:
      let roota = a.skipGenericAlias
      let rootf = f.lastSon.skipGenericAlias
      if a.kind == tyGenericInst and roota.base == rootf.base:
        for i in 1 .. rootf.sonsLen-2:
          let ff = rootf.sons[i]
          let aa = roota.sons[i]
          result = typeRel(c, ff, aa)
          if result == isNone: return
          if ff.kind == tyRange and result != isEqual: return isNone
      else:
        result = typeRel(c, rootf.lastSon, a)
      if result != isNone:
        put(c, f, a)
        result = isGeneric

  of tyGenericParam:
    var x = PType(idTableGet(c.bindings, f))
    if x == nil:
      if c.callee.kind == tyGenericBody and
         f.kind == tyGenericParam and not c.typedescMatched:
        # XXX: The fact that generic types currently use tyGenericParam for
        # their parameters is really a misnomer. tyGenericParam means "match
        # any value" and what we need is "match any type", which can be encoded
        # by a tyTypeDesc params. Unfortunately, this requires more substantial
        # changes in semtypinst and elsewhere.
        if tfWildcard in a.flags:
          result = isGeneric
        elif a.kind == tyTypeDesc:
          if f.sonsLen == 0:
            result = isGeneric
          else:
            internalAssert c.c.graph.config, a.sons != nil and a.sons.len > 0
            c.typedescMatched = true
            var aa = a
            while aa.kind in {tyTypeDesc, tyGenericParam} and aa.len > 0:
              aa = lastSon(aa)
            if aa.kind == tyGenericParam:
              return isGeneric
            result = typeRel(c, f.base, aa)
            if result > isGeneric: result = isGeneric
        else:
          result = isNone
      else:
        if f.sonsLen > 0 and f.sons[0].kind != tyNone:
          let oldInheritancePenalty = c.inheritancePenalty
          result = typeRel(c, f.lastSon, a, flags + {trDontBind})
          if doBind and result notin {isNone, isGeneric}:
            let concrete = concreteType(c, a)
            if concrete == nil: return isNone
            put(c, f, concrete)
          # bug #6526
          if result in {isEqual, isSubtype}:
            # 'T: Class' is a *better* match than just 'T'
            # but 'T: Subclass' is even better:
            c.inheritancePenalty = oldInheritancePenalty - c.inheritancePenalty -
                                  100 * ord(result == isEqual)
            result = isGeneric
        else:
          result = isGeneric

      if result == isGeneric:
        var concrete = a
        if tfWildcard in a.flags:
          a.sym.kind = skType
          a.flags.excl tfWildcard
        else:
          concrete = concreteType(c, a)
          if concrete == nil:
            return isNone
        if doBind:
          put(c, f, concrete)
      elif result > isGeneric:
        result = isGeneric
    elif a.kind == tyEmpty:
      result = isGeneric
    elif x.kind == tyGenericParam:
      result = isGeneric
    else:
      # Special type binding rule for numeric types.
      # See section "Generic type inference for numeric types" of the
      # manual for further details:
      when false:
        let rebinding = maxNumericType(x.skipTypes({tyRange}), a)
        if rebinding != nil:
          put(c, f, rebinding)
          result = isGeneric
        else:
          discard
      result = typeRel(c, x, a) # check if it fits
      if result > isGeneric: result = isGeneric
  of tyStatic:
    let prev = PType(idTableGet(c.bindings, f))
    if prev == nil:
      if aOrig.kind == tyStatic:
        result = typeRel(c, f.lastSon, a)
        if result != isNone and f.n != nil:
          if not exprStructuralEquivalent(f.n, aOrig.n):
            result = isNone
        if result != isNone: put(c, f, aOrig)
      elif aOrig.n != nil and aOrig.n.typ != nil:
        result = typeRel(c, f.lastSon, aOrig.n.typ)
        if result != isNone:
          var boundType = newTypeWithSons(c.c, tyStatic, @[aOrig.n.typ])
          boundType.n = aOrig.n
          put(c, f, boundType)
      else:
        result = isNone
    elif prev.kind == tyStatic:
      if aOrig.kind == tyStatic:
        result = typeRel(c, prev.lastSon, a)
        if result != isNone and prev.n != nil:
          if not exprStructuralEquivalent(prev.n, aOrig.n):
            result = isNone
      else: result = isNone
    else:
      # XXX endless recursion?
      #result = typeRel(c, prev, aOrig)
      result = isNone

  of tyInferred:
    let prev = f.previouslyInferred
    if prev != nil:
      result = typeRel(c, prev, a)
    else:
      result = typeRel(c, f.base, a)
      if result != isNone:
        c.inferredTypes.safeAdd f
        f.sons.add a

  of tyTypeDesc:
    var prev = PType(idTableGet(c.bindings, f))
    if prev == nil:
      # proc foo(T: typedesc, x: T)
      # when `f` is an unresolved typedesc, `a` could be any
      # type, so we should not perform this check earlier
      if a.kind != tyTypeDesc: return isNone

      if f.base.kind == tyNone:
        result = isGeneric
      else:
        result = typeRel(c, f.base, a.base)

      if result != isNone:
        put(c, f, a)
    else:
      if tfUnresolved in f.flags:
        result = typeRel(c, prev.base, a)
      elif a.kind == tyTypeDesc:
        result = typeRel(c, prev.base, a.base)
      else:
        result = isNone

  of tyStmt:
    if aOrig != nil and tfOldSchoolExprStmt notin f.flags:
      put(c, f, aOrig)
    result = isGeneric

  of tyProxy:
    result = isEqual

  of tyFromExpr:
    # fix the expression, so it contains the already instantiated types
    if f.n == nil or f.n.kind == nkEmpty: return isGeneric
    let reevaluated = tryResolvingStaticExpr(c, f.n)
    case reevaluated.typ.kind
    of tyTypeDesc:
      result = typeRel(c, a, reevaluated.typ.base)
    of tyStatic:
      result = typeRel(c, a, reevaluated.typ.base)
      if result != isNone and reevaluated.typ.n != nil:
        if not exprStructuralEquivalent(aOrig.n, reevaluated.typ.n):
          result = isNone
    else:
      localError(c.c.graph.config, f.n.info, "type expected")
      result = isNone

  of tyNone:
    if a.kind == tyNone: result = isEqual
  else:
    internalError c.c.graph.config, " unknown type kind " & $f.kind

proc cmpTypes*(c: PContext, f, a: PType): TTypeRelation =
  var m: TCandidate
  initCandidate(c, m, f)
  result = typeRel(m, f, a)

proc getInstantiatedType(c: PContext, arg: PNode, m: TCandidate,
                         f: PType): PType =
  result = PType(idTableGet(m.bindings, f))
  if result == nil:
    result = generateTypeInstance(c, m.bindings, arg, f)
  if result == nil:
    internalError(c.graph.config, arg.info, "getInstantiatedType")
    result = errorType(c)

proc implicitConv(kind: TNodeKind, f: PType, arg: PNode, m: TCandidate,
                  c: PContext): PNode =
  result = newNodeI(kind, arg.info)
  if containsGenericType(f):
    if not m.hasFauxMatch:
      result.typ = getInstantiatedType(c, arg, m, f)
    else:
      result.typ = errorType(c)
  else:
    result.typ = f
  if result.typ == nil: internalError(c.graph.config, arg.info, "implicitConv")
  addSon(result, c.graph.emptyNode)
  addSon(result, arg)

proc userConvMatch(c: PContext, m: var TCandidate, f, a: PType,
                   arg: PNode): PNode =
  result = nil
  for i in countup(0, len(c.converters) - 1):
    var src = c.converters[i].typ.sons[1]
    var dest = c.converters[i].typ.sons[0]
    # for generic type converters we need to check 'src <- a' before
    # 'f <- dest' in order to not break the unification:
    # see tests/tgenericconverter:
    let srca = typeRel(m, src, a)
    if srca notin {isEqual, isGeneric}: continue

    let destIsGeneric = containsGenericType(dest)
    if destIsGeneric:
      dest = generateTypeInstance(c, m.bindings, arg, dest)
    let fdest = typeRel(m, f, dest)
    if fdest in {isEqual, isGeneric}:
      markUsed(c.config, arg.info, c.converters[i], c.graph.usageSym)
      var s = newSymNode(c.converters[i])
      s.typ = c.converters[i].typ
      s.info = arg.info
      result = newNodeIT(nkHiddenCallConv, arg.info, dest)
      addSon(result, s)
      addSon(result, copyTree(arg))
      inc(m.convMatches)
      m.genericConverter = srca == isGeneric or destIsGeneric
      return result

proc localConvMatch(c: PContext, m: var TCandidate, f, a: PType,
                    arg: PNode): PNode =
  # arg.typ can be nil in 'suggest':
  if isNil(arg.typ): return nil

  # sem'checking for 'echo' needs to be re-entrant:
  # XXX we will revisit this issue after 0.10.2 is released
  if f == arg.typ and arg.kind == nkHiddenStdConv: return arg

  var call = newNodeI(nkCall, arg.info)
  call.add(f.n.copyTree)
  call.add(arg.copyTree)
  result = c.semExpr(c, call)
  if result != nil:
    if result.typ == nil: return nil
    # resulting type must be consistent with the other arguments:
    var r = typeRel(m, f.sons[0], result.typ)
    if r < isGeneric: return nil
    if result.kind == nkCall: result.kind = nkHiddenCallConv
    inc(m.convMatches)
    if r == isGeneric:
      result.typ = getInstantiatedType(c, arg, m, base(f))
    m.baseTypeMatch = true

proc incMatches(m: var TCandidate; r: TTypeRelation; convMatch = 1) =
  case r
  of isConvertible, isIntConv: inc(m.convMatches, convMatch)
  of isSubtype, isSubrange: inc(m.subtypeMatches)
  of isGeneric, isInferred, isBothMetaConvertible: inc(m.genericMatches)
  of isFromIntLit: inc(m.intConvMatches, 256)
  of isInferredConvertible:
    inc(m.convMatches)
  of isEqual: inc(m.exactMatches)
  of isNone: discard

template matchesVoidProc(t: PType): bool =
  (t.kind == tyProc and t.len == 1 and t.sons[0] == nil) or
    (t.kind == tyBuiltInTypeClass and t.sons[0].kind == tyProc)

proc paramTypesMatchAux(m: var TCandidate, f, a: PType,
                        argSemantized, argOrig: PNode): PNode =
  var
    fMaybeStatic = f.skipTypes({tyDistinct})
    arg = argSemantized
    a = a
    c = m.c

  if tfHasStatic in fMaybeStatic.flags:
    # XXX: When implicit statics are the default
    # this will be done earlier - we just have to
    # make sure that static types enter here

    # XXX: weaken tyGenericParam and call it tyGenericPlaceholder
    # and finally start using tyTypedesc for generic types properly.
    if a.kind == tyGenericParam and tfWildcard in a.flags:
      a.assignType(f)
      # put(m.bindings, f, a)
      return argSemantized

    if a.kind == tyStatic:
      if m.callee.kind == tyGenericBody and
         a.n == nil and
         tfGenericTypeParam notin a.flags:
        return newNodeIT(nkType, argOrig.info, makeTypeFromExpr(c, arg))
    else:
      var evaluated = c.semTryConstExpr(c, arg)
      if evaluated != nil:
        arg.typ = newTypeS(tyStatic, c)
        arg.typ.sons = @[evaluated.typ]
        arg.typ.n = evaluated
        a = arg.typ
      else:
        if m.callee.kind == tyGenericBody:
          if f.kind == tyStatic and typeRel(m, f.base, a) != isNone:
            result = makeStaticExpr(m.c, arg)
            result.typ.flags.incl tfUnresolved
            result.typ.n = arg
            return

  var r = typeRel(m, f, a)

  # This special typing rule for macros and templates is not documented
  # anywhere and breaks symmetry. It's hard to get rid of though, my
  # custom seqs example fails to compile without this:
  if r != isNone and m.calleeSym != nil and
    m.calleeSym.kind in {skMacro, skTemplate}:
    # XXX: duplicating this is ugly, but we cannot (!) move this
    # directly into typeRel using return-like templates
    incMatches(m, r)
    if f.kind == tyStmt:
      return arg
    elif f.kind == tyTypeDesc:
      return arg
    elif f.kind == tyStatic and arg.typ.n != nil:
      return arg.typ.n
    else:
      return argSemantized # argOrig

  # If r == isBothMetaConvertible then we rerun typeRel.
  # bothMetaCounter is for safety to avoid any infinite loop,
  #  I don't have any example when it is needed.
  # lastBindingsLenth is used to check whether m.bindings remains the same,
  #  because in that case there is no point in continuing.
  var bothMetaCounter = 0
  var lastBindingsLength = -1
  while r == isBothMetaConvertible and
      lastBindingsLength != m.bindings.counter and
      bothMetaCounter < 100:
    lastBindingsLength = m.bindings.counter
    inc(bothMetaCounter)
    if arg.kind in {nkProcDef, nkFuncDef, nkIteratorDef} + nkLambdaKinds:
      result = c.semInferredLambda(c, m.bindings, arg)
    elif arg.kind != nkSym:
      return nil
    else:
      let inferred = c.semGenerateInstance(c, arg.sym, m.bindings, arg.info)
      result = newSymNode(inferred, arg.info)
    inc(m.convMatches)
    arg = result
    r = typeRel(m, f, arg.typ)

  case r
  of isConvertible:
    inc(m.convMatches)
    result = implicitConv(nkHiddenStdConv, f, arg, m, c)
  of isIntConv:
    # I'm too lazy to introduce another ``*matches`` field, so we conflate
    # ``isIntConv`` and ``isIntLit`` here:
    inc(m.intConvMatches)
    result = implicitConv(nkHiddenStdConv, f, arg, m, c)
  of isSubtype:
    inc(m.subtypeMatches)
    if f.kind == tyTypeDesc:
      result = arg
    else:
      result = implicitConv(nkHiddenSubConv, f, arg, m, c)
  of isSubrange:
    inc(m.subtypeMatches)
    if f.kind == tyVar:
      result = arg
    else:
      result = implicitConv(nkHiddenStdConv, f, arg, m, c)
  of isInferred, isInferredConvertible:
    if arg.kind in {nkProcDef, nkFuncDef, nkIteratorDef} + nkLambdaKinds:
      result = c.semInferredLambda(c, m.bindings, arg)
    elif arg.kind != nkSym:
      return nil
    else:
      let inferred = c.semGenerateInstance(c, arg.sym, m.bindings, arg.info)
      result = newSymNode(inferred, arg.info)
    if r == isInferredConvertible:
      inc(m.convMatches)
      result = implicitConv(nkHiddenStdConv, f, result, m, c)
    else:
      inc(m.genericMatches)
  of isGeneric:
    inc(m.genericMatches)
    if arg.typ == nil:
      result = arg
    elif skipTypes(arg.typ, abstractVar-{tyTypeDesc}).kind == tyTuple or
         m.inheritancePenalty > 0:
      result = implicitConv(nkHiddenSubConv, f, arg, m, c)
    elif arg.typ.isEmptyContainer:
      result = arg.copyTree
      result.typ = getInstantiatedType(c, arg, m, f)
    else:
      result = arg
  of isBothMetaConvertible:
    # This is the result for the 101th time.
    result = nil
  of isFromIntLit:
    # too lazy to introduce another ``*matches`` field, so we conflate
    # ``isIntConv`` and ``isIntLit`` here:
    inc(m.intConvMatches, 256)
    result = implicitConv(nkHiddenStdConv, f, arg, m, c)
  of isEqual:
    inc(m.exactMatches)
    result = arg
    if skipTypes(f, abstractVar-{tyTypeDesc}).kind in {tyTuple}:
      result = implicitConv(nkHiddenSubConv, f, arg, m, c)
  of isNone:
    # do not do this in ``typeRel`` as it then can't infer T in ``ref T``:
    if a.kind in {tyProxy, tyUnknown}:
      inc(m.genericMatches)
      m.fauxMatch = a.kind
      return arg
    elif a.kind == tyVoid and f.matchesVoidProc and argOrig.kind == nkStmtList:
      # lift do blocks without params to lambdas
      let p = c.graph
      let lifted = c.semExpr(c, newProcNode(nkDo, argOrig.info, body = argOrig,
          params = p.emptyNode, name = p.emptyNode, pattern = p.emptyNode,
          genericParams = p.emptyNode, pragmas = p.emptyNode, exceptions = p.emptyNode), {})
      if f.kind == tyBuiltInTypeClass:
        inc m.genericMatches
        put(m, f, lifted.typ)
      inc m.convMatches
      return implicitConv(nkHiddenStdConv, f, lifted, m, c)
    result = userConvMatch(c, m, f, a, arg)
    # check for a base type match, which supports varargs[T] without []
    # constructor in a call:
    if result == nil and f.kind == tyVarargs:
      if f.n != nil:
        result = localConvMatch(c, m, f, a, arg)
      else:
        r = typeRel(m, base(f), a)
        if r >= isGeneric:
          inc(m.convMatches)
          result = copyTree(arg)
          if r == isGeneric:
            result.typ = getInstantiatedType(c, arg, m, base(f))
          m.baseTypeMatch = true
        else:
          result = userConvMatch(c, m, base(f), a, arg)
          if result != nil: m.baseTypeMatch = true

proc paramTypesMatch*(m: var TCandidate, f, a: PType,
                      arg, argOrig: PNode): PNode =
  if arg == nil or arg.kind notin nkSymChoices:
    result = paramTypesMatchAux(m, f, a, arg, argOrig)
  else:
    # CAUTION: The order depends on the used hashing scheme. Thus it is
    # incorrect to simply use the first fitting match. However, to implement
    # this correctly is inefficient. We have to copy `m` here to be able to
    # roll back the side effects of the unification algorithm.
    let c = m.c
    var x, y, z: TCandidate
    initCandidate(c, x, m.callee)
    initCandidate(c, y, m.callee)
    initCandidate(c, z, m.callee)
    x.calleeSym = m.calleeSym
    y.calleeSym = m.calleeSym
    z.calleeSym = m.calleeSym
    var best = -1
    for i in 0 ..< arg.len:
      if arg.sons[i].sym.kind in {skProc, skFunc, skMethod, skConverter,
                                  skIterator, skMacro, skTemplate}:
        copyCandidate(z, m)
        z.callee = arg.sons[i].typ
        if tfUnresolved in z.callee.flags: continue
        z.calleeSym = arg.sons[i].sym
        #if arg.sons[i].sym.name.s == "cmp":
        #  ggDebug = true
        #  echo "CALLLEEEEEEEE A ", typeToString(z.callee)
        # XXX this is still all wrong: (T, T) should be 2 generic matches
        # and  (int, int) 2 exact matches, etc. Essentially you cannot call
        # typeRel here and expect things to work!
        let r = typeRel(z, f, arg.sons[i].typ)
        incMatches(z, r, 2)
        #if arg.sons[i].sym.name.s == "cmp": # and arg.info.line == 606:
        #  echo "M ", r, " ", arg.info, " ", typeToString(arg.sons[i].sym.typ)
        #  writeMatches(z)
        if r != isNone:
          z.state = csMatch
          case x.state
          of csEmpty, csNoMatch:
            x = z
            best = i
          of csMatch:
            let cmp = cmpCandidates(x, z)
            if cmp < 0:
              best = i
              x = z
            elif cmp == 0:
              y = z           # z is as good as x

    if x.state == csEmpty:
      result = nil
    elif y.state == csMatch and cmpCandidates(x, y) == 0:
      if x.state != csMatch:
        internalError(m.c.graph.config, arg.info, "x.state is not csMatch")
      # ambiguous: more than one symbol fits!
      # See tsymchoice_for_expr as an example. 'f.kind == tyExpr' should match
      # anyway:
      if f.kind in {tyExpr, tyStmt}: result = arg
      else: result = nil
    else:
      # only one valid interpretation found:
      markUsed(m.c.config, arg.info, arg.sons[best].sym, m.c.graph.usageSym)
      styleCheckUse(arg.info, arg.sons[best].sym)
      result = paramTypesMatchAux(m, f, arg.sons[best].typ, arg.sons[best],
                                  argOrig)

proc setSon(father: PNode, at: int, son: PNode) =
  let oldLen = father.len
  if oldLen <= at:
    setLen(father.sons, at + 1)
  father.sons[at] = son
  # insert potential 'void' parameters:
  #for i in oldLen ..< at:
  #  father.sons[i] = newNodeIT(nkEmpty, son.info, getSysType(tyVoid))

# we are allowed to modify the calling node in the 'prepare*' procs:
proc prepareOperand(c: PContext; formal: PType; a: PNode): PNode =
  if formal.kind == tyExpr and formal.len != 1:
    # {tyTypeDesc, tyExpr, tyStmt, tyProxy}:
    # a.typ == nil is valid
    result = a
  elif a.typ.isNil:
    # XXX This is unsound! 'formal' can differ from overloaded routine to
    # overloaded routine!
    let flags = {efDetermineType, efAllowStmt}
                #if formal.kind == tyIter: {efDetermineType, efWantIterator}
                #else: {efDetermineType, efAllowStmt}
                #elif formal.kind == tyStmt: {efDetermineType, efWantStmt}
                #else: {efDetermineType}
    result = c.semOperand(c, a, flags)
  else:
    result = a
    considerGenSyms(c, result)

proc prepareOperand(c: PContext; a: PNode): PNode =
  if a.typ.isNil:
    result = c.semOperand(c, a, {efDetermineType})
  else:
    result = a
    considerGenSyms(c, result)

proc prepareNamedParam(a: PNode; c: PContext) =
  if a.sons[0].kind != nkIdent:
    var info = a.sons[0].info
    a.sons[0] = newIdentNode(considerQuotedIdent(c, a.sons[0]), info)

proc arrayConstr(c: PContext, n: PNode): PType =
  result = newTypeS(tyArray, c)
  rawAddSon(result, makeRangeType(c, 0, 0, n.info))
  addSonSkipIntLit(result, skipTypes(n.typ,
      {tyGenericInst, tyVar, tyLent, tyOrdinal}))

proc arrayConstr(c: PContext, info: TLineInfo): PType =
  result = newTypeS(tyArray, c)
  rawAddSon(result, makeRangeType(c, 0, -1, info))
  rawAddSon(result, newTypeS(tyEmpty, c)) # needs an empty basetype!

proc incrIndexType(t: PType) =
  assert t.kind == tyArray
  inc t.sons[0].n.sons[1].intVal

template isVarargsUntyped(x): untyped =
  x.kind == tyVarargs and x.sons[0].kind == tyExpr and
    tfOldSchoolExprStmt notin x.sons[0].flags

proc matchesAux(c: PContext, n, nOrig: PNode,
                m: var TCandidate, marker: var IntSet) =
  template checkConstraint(n: untyped) {.dirty.} =
    if not formal.constraint.isNil:
      if matchNodeKinds(formal.constraint, n):
        # better match over other routines with no such restriction:
        inc(m.genericMatches, 100)
      else:
        m.state = csNoMatch
        return
    if formal.typ.kind == tyVar:
      if not n.isLValue:
        m.state = csNoMatch
        m.mutabilityProblem = uint8(f-1)
        return

  var
    # iterates over formal parameters
    f = if m.callee.kind != tyGenericBody: 1
        else: 0
    # iterates over the actual given arguments
    a = 1

  m.state = csMatch # until proven otherwise
  m.call = newNodeI(n.kind, n.info)
  m.call.typ = base(m.callee) # may be nil
  var formalLen = m.callee.n.len
  addSon(m.call, copyTree(n.sons[0]))
  var container: PNode = nil # constructed container
  var formal: PSym = if formalLen > 1: m.callee.n.sons[1].sym else: nil

  while a < n.len:
    if a >= formalLen-1 and f < formalLen and m.callee.n[f].typ.isVarargsUntyped:
      formal = m.callee.n.sons[f].sym
      incl(marker, formal.position)
      if container.isNil:
        container = newNodeIT(nkArgList, n.sons[a].info, arrayConstr(c, n.info))
        setSon(m.call, formal.position + 1, container)
      else:
        incrIndexType(container.typ)
      addSon(container, n.sons[a])
    elif n.sons[a].kind == nkExprEqExpr:
      # named param
      # check if m.callee has such a param:
      prepareNamedParam(n.sons[a], c)
      if n.sons[a].sons[0].kind != nkIdent:
        localError(c.config, n.sons[a].info, "named parameter has to be an identifier")
        m.state = csNoMatch
        return
      formal = getSymFromList(m.callee.n, n.sons[a].sons[0].ident, 1)
      if formal == nil:
        # no error message!
        m.state = csNoMatch
        return
      if containsOrIncl(marker, formal.position):
        # already in namedParams, so no match
        # we used to produce 'errCannotBindXTwice' here but see
        # bug #3836 of why that is not sound (other overload with
        # different parameter names could match later on):
        when false: localError(n.sons[a].info, errCannotBindXTwice, formal.name.s)
        m.state = csNoMatch
        return
      m.baseTypeMatch = false
      n.sons[a].sons[1] = prepareOperand(c, formal.typ, n.sons[a].sons[1])
      n.sons[a].typ = n.sons[a].sons[1].typ
      var arg = paramTypesMatch(m, formal.typ, n.sons[a].typ,
                                n.sons[a].sons[1], n.sons[a].sons[1])
      if arg == nil:
        m.state = csNoMatch
        return
      checkConstraint(n.sons[a].sons[1])
      if m.baseTypeMatch:
        #assert(container == nil)
        container = newNodeIT(nkBracket, n.sons[a].info, arrayConstr(c, arg))
        addSon(container, arg)
        setSon(m.call, formal.position + 1, container)
        if f != formalLen - 1: container = nil
      else:
        setSon(m.call, formal.position + 1, arg)
      inc f
    else:
      # unnamed param
      if f >= formalLen:
        # too many arguments?
        if tfVarargs in m.callee.flags:
          # is ok... but don't increment any counters...
          # we have no formal here to snoop at:
          n.sons[a] = prepareOperand(c, n.sons[a])
          if skipTypes(n.sons[a].typ, abstractVar-{tyTypeDesc}).kind==tyString:
            addSon(m.call, implicitConv(nkHiddenStdConv,
                  getSysType(c.graph, n.sons[a].info, tyCString),
                  copyTree(n.sons[a]), m, c))
          else:
            addSon(m.call, copyTree(n.sons[a]))
        elif formal != nil and formal.typ.kind == tyVarargs:
          # beware of the side-effects in 'prepareOperand'! So only do it for
          # varargs matching. See tests/metatype/tstatic_overloading.
          m.baseTypeMatch = false
          incl(marker, formal.position)
          n.sons[a] = prepareOperand(c, formal.typ, n.sons[a])
          var arg = paramTypesMatch(m, formal.typ, n.sons[a].typ,
                                    n.sons[a], nOrig.sons[a])
          if arg != nil and m.baseTypeMatch and container != nil:
            addSon(container, arg)
            incrIndexType(container.typ)
            checkConstraint(n.sons[a])
          else:
            m.state = csNoMatch
            return
        else:
          m.state = csNoMatch
          return
      else:
        if m.callee.n.sons[f].kind != nkSym:
          internalError(c.config, n.sons[a].info, "matches")
          return
        formal = m.callee.n.sons[f].sym
        if containsOrIncl(marker, formal.position) and container.isNil:
          # already in namedParams: (see above remark)
          when false: localError(n.sons[a].info, errCannotBindXTwice, formal.name.s)
          m.state = csNoMatch
          return

        if formal.typ.isVarargsUntyped:
          if container.isNil:
            container = newNodeIT(nkArgList, n.sons[a].info, arrayConstr(c, n.info))
            setSon(m.call, formal.position + 1, container)
          else:
            incrIndexType(container.typ)
          addSon(container, n.sons[a])
        else:
          m.baseTypeMatch = false
          n.sons[a] = prepareOperand(c, formal.typ, n.sons[a])
          var arg = paramTypesMatch(m, formal.typ, n.sons[a].typ,
                                    n.sons[a], nOrig.sons[a])
          if arg == nil:
            m.state = csNoMatch
            m.firstMismatch = f
            return
          if m.baseTypeMatch:
            #assert(container == nil)
            if container.isNil:
              container = newNodeIT(nkBracket, n.sons[a].info, arrayConstr(c, arg))
              container.typ.flags.incl tfVarargs
            else:
              incrIndexType(container.typ)
            addSon(container, arg)
            setSon(m.call, formal.position + 1,
                   implicitConv(nkHiddenStdConv, formal.typ, container, m, c))
            #if f != formalLen - 1: container = nil

            # pick the formal from the end, so that 'x, y, varargs, z' works:
            f = max(f, formalLen - n.len + a + 1)
          else:
            setSon(m.call, formal.position + 1, arg)
            inc(f)
            container = nil
        checkConstraint(n.sons[a])
    inc(a)

proc semFinishOperands*(c: PContext, n: PNode) =
  # this needs to be called to ensure that after overloading resolution every
  # argument has been sem'checked:
  for i in 1 ..< n.len:
    n.sons[i] = prepareOperand(c, n.sons[i])

proc partialMatch*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
  # for 'suggest' support:
  var marker = initIntSet()
  matchesAux(c, n, nOrig, m, marker)

proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
  if m.magic in {mArrGet, mArrPut}:
    m.state = csMatch
    m.call = n
    return
  var marker = initIntSet()
  matchesAux(c, n, nOrig, m, marker)
  if m.state == csNoMatch: return
  # check that every formal parameter got a value:
  var f = 1
  while f < sonsLen(m.callee.n):
    var formal = m.callee.n.sons[f].sym
    if not containsOrIncl(marker, formal.position):
      if formal.ast == nil:
        if formal.typ.kind == tyVarargs:
          var container = newNodeIT(nkBracket, n.info, arrayConstr(c, n.info))
          setSon(m.call, formal.position + 1,
                 implicitConv(nkHiddenStdConv, formal.typ, container, m, c))
        else:
          # no default value
          m.state = csNoMatch
          m.firstMismatch = f
          break
      else:
        # use default value:
        var def = copyTree(formal.ast)
        if def.kind == nkNilLit:
          def = implicitConv(nkHiddenStdConv, formal.typ, def, m, c)
        if {tfImplicitTypeParam, tfGenericTypeParam} * formal.typ.flags != {}:
          put(m, formal.typ, def.typ)
        setSon(m.call, formal.position + 1, def)
    inc(f)
  # forget all inferred types if the overload matching failed
  if m.state == csNoMatch:
    for t in m.inferredTypes:
      if t.sonsLen > 1: t.sons.setLen 1

proc argtypeMatches*(c: PContext, f, a: PType): bool =
  var m: TCandidate
  initCandidate(c, m, f)
  let res = paramTypesMatch(m, f, a, c.graph.emptyNode, nil)
  #instantiateGenericConverters(c, res, m)
  # XXX this is used by patterns.nim too; I think it's better to not
  # instantiate generic converters for that
  result = res != nil

proc instTypeBoundOp*(c: PContext; dc: PSym; t: PType; info: TLineInfo;
                      op: TTypeAttachedOp; col: int): PSym {.procvar.} =
  var m: TCandidate
  initCandidate(c, m, dc.typ)
  if col >= dc.typ.len:
    localError(c.config, info, "cannot instantiate: '" & dc.name.s & "'")
    return nil
  var f = dc.typ.sons[col]

  if op == attachedDeepCopy:
    if f.kind in {tyRef, tyPtr}: f = f.lastSon
  else:
    if f.kind == tyVar: f = f.lastSon
  if typeRel(m, f, t) == isNone:
    localError(c.config, info, "cannot instantiate: '" & dc.name.s & "'")
  else:
    result = c.semGenerateInstance(c, dc, m.bindings, info)
    if op == attachedDeepCopy:
      assert sfFromGeneric in result.flags

include suggest

when not declared(tests):
  template tests(s: untyped) = discard

tests:
  var dummyOwner = newSym(skModule, getIdent("test_module"), nil, UnknownLineInfo())

  proc `|` (t1, t2: PType): PType =
    result = newType(tyOr, dummyOwner)
    result.rawAddSon(t1)
    result.rawAddSon(t2)

  proc `&` (t1, t2: PType): PType =
    result = newType(tyAnd, dummyOwner)
    result.rawAddSon(t1)
    result.rawAddSon(t2)

  proc `!` (t: PType): PType =
    result = newType(tyNot, dummyOwner)
    result.rawAddSon(t)

  proc seq(t: PType): PType =
    result = newType(tySequence, dummyOwner)
    result.rawAddSon(t)

  proc array(x: int, t: PType): PType =
    result = newType(tyArray, dummyOwner)

    var n = newNodeI(nkRange, UnknownLineInfo())
    addSon(n, newIntNode(nkIntLit, 0))
    addSon(n, newIntNode(nkIntLit, x))
    let range = newType(tyRange, dummyOwner)

    result.rawAddSon(range)
    result.rawAddSon(t)

  suite "type classes":
    let
      int = newType(tyInt, dummyOwner)
      float = newType(tyFloat, dummyOwner)
      string = newType(tyString, dummyOwner)
      ordinal = newType(tyOrdinal, dummyOwner)
      any = newType(tyAnything, dummyOwner)
      number = int | float

    var TFoo = newType(tyObject, dummyOwner)
    TFoo.sym = newSym(skType, getIdent"TFoo", dummyOwner, UnknownLineInfo())

    var T1 = newType(tyGenericParam, dummyOwner)
    T1.sym = newSym(skType, getIdent"T1", dummyOwner, UnknownLineInfo())
    T1.sym.position = 0

    var T2 = newType(tyGenericParam, dummyOwner)
    T2.sym = newSym(skType, getIdent"T2", dummyOwner, UnknownLineInfo())
    T2.sym.position = 1

    setup:
      var c: TCandidate
      initCandidate(nil, c, nil)

    template yes(x, y) =
      test astToStr(x) & " is " & astToStr(y):
        check typeRel(c, y, x) == isGeneric

    template no(x, y) =
      test astToStr(x) & " is not " & astToStr(y):
        check typeRel(c, y, x) == isNone

    yes seq(any), array(10, int) | seq(any)
    # Sure, seq[any] is directly included

    yes seq(int), seq(any)
    yes seq(int), seq(number)
    # Sure, the int sequence is certainly
    # part of the number sequences (and all sequences)

    no seq(any), seq(float)
    # Nope, seq[any] includes types that are not seq[float] (e.g. seq[int])

    yes seq(int|string), seq(any)
    # Sure

    yes seq(int&string), seq(any)
    # Again

    yes seq(int&string), seq(int)
    # A bit more complicated
    # seq[int&string] is not a real type, but it's analogous to
    # seq[Sortable and Iterable], which is certainly a subset of seq[Sortable]

    no seq(int|string), seq(int|float)
    # Nope, seq[string] is not included in not included in
    # the seq[int|float] set

    no seq(!(int|string)), seq(string)
    # A sequence that is neither seq[int] or seq[string]
    # is obviously not seq[string]

    no seq(!int), seq(number)
    # Now your head should start to hurt a bit
    # A sequence that is not seq[int] is not necessarily a number sequence
    # it could well be seq[string] for example

    yes seq(!(int|string)), seq(!string)
    # all sequnece types besides seq[int] and seq[string]
    # are subset of all sequence types that are not seq[string]

    no seq(!(int|string)), seq(!(string|TFoo))
    # Nope, seq[TFoo] is included in the first set, but not in the second

    no seq(!string), seq(!number)
    # Nope, seq[int] in included in the first set, but not in the second

    yes seq(!number), seq(any)
    yes seq(!int), seq(any)
    no seq(any), seq(!any)
    no seq(!int), seq(!any)

    yes int, ordinal
    no  string, ordinal