better type inference for numerical types; prerequisitive for version 1

compiler/sigmatch.nim

@@ -913,6 +913,25 @@ proc isCovariantPtr(c: var TCandidate, f, a: PType): bool =
   else:
     return 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:
@@ -1602,9 +1621,16 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
     elif x.kind == tyGenericParam:
       result = isGeneric
     else:
-      result = typeRel(c, x, a) # check if it fits
-      if result > isGeneric: result = isGeneric
-
+      # Special type binding rule for numeric types.
+      # See section "Generic type inference for numeric types" of the
+      # manual for further details:
+      let rebinding = maxNumericType(x.skipTypes({tyRange}), a)
+      if rebinding != nil:
+        put(c, f, rebinding)
+        result = isGeneric
+      else:
+        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: