improve the error messages regarding type mismatches in overloading resolution

compiler/semcall.nim

@@ -106,6 +106,7 @@ proc pickBestCandidate(c: PContext, headSymbol: PNode,
         errors.safeAdd(CandidateError(
           sym: sym,
           unmatchedVarParam: int z.mutabilityProblem,
+          firstMismatch: z.firstMismatch,
           diagnostics: z.diagnostics))
     else:
       # Symbol table has been modified. Restart and pre-calculate all syms
@@ -154,7 +155,20 @@ proc presentFailedCandidates(c: PContext, n: PNode, errors: CandidateErrors):
     else:
       add(candidates, err.sym.getProcHeader(prefer))
     add(candidates, "\n")
-    if err.unmatchedVarParam != 0 and err.unmatchedVarParam < n.len:
+    if err.firstMismatch != 0 and n.len > 2:
+      add(candidates, "first type mismatch at position: " & $err.firstMismatch &
+        "\nrequired type: ")
+      if err.firstMismatch < err.sym.typ.len:
+        candidates.add typeToString(err.sym.typ.sons[err.firstMismatch])
+      else:
+        candidates.add "none"
+      if err.firstMismatch < n.len:
+        candidates.add "\nbut expression '"
+        candidates.add renderTree(n[err.firstMismatch])
+        candidates.add "' is of type: "
+        candidates.add typeToString(n[err.firstMismatch].typ)
+      candidates.add "\n"
+    elif err.unmatchedVarParam != 0 and err.unmatchedVarParam < n.len:
       add(candidates, "for a 'var' type a variable needs to be passed, but '" &
                       renderTree(n[err.unmatchedVarParam]) & "' is immutable\n")
     for diag in err.diagnostics:
@@ -189,7 +203,7 @@ proc bracketNotFoundError(c: PContext; n: PNode) =
   while symx != nil:
     if symx.kind in routineKinds:
       errors.add(CandidateError(sym: symx,
-                                unmatchedVarParam: 0,
+                                unmatchedVarParam: 0, firstMismatch: 0,
                                 diagnostics: nil))
     symx = nextOverloadIter(o, c, headSymbol)
   if errors.len == 0:

compiler/sigmatch.nim

@@ -24,7 +24,7 @@ type
 
   CandidateError* = object
     sym*: PSym
-    unmatchedVarParam*: int
+    unmatchedVarParam*, firstMismatch*: int
     diagnostics*: seq[string]
 
   CandidateErrors* = seq[CandidateError]
@@ -68,6 +68,8 @@ type
                               # 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
 
   TTypeRelFlag* = enum
     trDontBind
@@ -2249,6 +2251,7 @@ proc matchesAux(c: PContext, n, nOrig: PNode,
                                     n.sons[a], nOrig.sons[a])
           if arg == nil:
             m.state = csNoMatch
+            m.firstMismatch = f
             return
           if m.baseTypeMatch:
             #assert(container == nil)
@@ -2303,6 +2306,7 @@ proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
         else:
           # no default value
           m.state = csNoMatch
+          m.firstMismatch = f
           break
       else:
         # use default value:

tests/concepts/t3330.nim

@@ -1,25 +1,48 @@
 discard """
 errormsg: "type mismatch: got (Bar[system.int])"
 nimout: '''
-t3330.nim(40, 4) Error: type mismatch: got (Bar[system.int])
+t3330.nim(63, 4) Error: type mismatch: got (Bar[system.int])
 but expected one of:
 proc test(foo: Foo[int])
-t3330.nim(25, 8) Hint: Non-matching candidates for add(k, string, T)
+t3330.nim(48, 8) Hint: Non-matching candidates for add(k, string, T)
 proc add(x: var string; y: string)
+first type mismatch at position: 1
+required type: var string
+but expression 'k' is of type: Alias
 proc add(x: var string; y: char)
+first type mismatch at position: 1
+required type: var string
+but expression 'k' is of type: Alias
 proc add(result: var string; x: int64)
+first type mismatch at position: 1
+required type: var string
+but expression 'k' is of type: Alias
 proc add(result: var string; x: float)
+first type mismatch at position: 1
+required type: var string
+but expression 'k' is of type: Alias
 proc add(x: var string; y: cstring)
+first type mismatch at position: 1
+required type: var string
+but expression 'k' is of type: Alias
 proc add[T](x: var seq[T]; y: openArray[T])
+first type mismatch at position: 1
+required type: var seq[T]
+but expression 'k' is of type: Alias
 proc add[T](x: var seq[T]; y: T)
+first type mismatch at position: 1
+required type: var seq[T]
+but expression 'k' is of type: Alias
 
-t3330.nim(25, 8) template/generic instantiation from here
+t3330.nim(48, 8) template/generic instantiation from here
-t3330.nim(32, 6) Foo: 'bar.value' cannot be assigned to
+t3330.nim(55, 6) Foo: 'bar.value' cannot be assigned to
-t3330.nim(25, 8) template/generic instantiation from here
+t3330.nim(48, 8) template/generic instantiation from here
-t3330.nim(33, 6) Foo: 'bar.x' cannot be assigned to
+t3330.nim(56, 6) Foo: 'bar.x' cannot be assigned to
-'''
+
+expression: test(bar)'''
 """
 
+
 type
   Foo[T] = concept k
     add(k, string, T)

tests/errmsgs/tdetailed_position.nim

@@ -0,0 +1,22 @@
+
+discard """
+cmd: "nim check $file"
+errormsg: "type mismatch: got (int literal(1), int literal(2), int literal(3))"
+nimout: '''
+but expected one of:
+proc main(a, b, c: string)
+first type mismatch at position: 1
+required type: string
+but expression '1' is of type: int literal(1)
+
+expression: main(1, 2, 3)
+'''
+"""
+
+const
+  myconst = "abcdefghijklmnopqrstuvwxyz"
+
+proc main(a, b, c: string) {.deprecated: "use foo " & "instead " & myconst.} =
+  return
+
+main(1, 2, 3)