some steps to improve the type mismatches with the new

generic instantiation logic

compiler/semtypes.nim

@@ -828,7 +828,10 @@ proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
     matches(c, n, copyTree(n), m)
     
     if m.state != csMatch:
-      LocalError(n.info, errWrongNumberOfArguments)
+      var err = "cannot instantiate " & typeToString(s.typ) & "\n" &
+                "got: (" & describeArgs(c, n) & ")\n" &
+                "but expected: (" & describeArgs(c, s.typ.n, 0) & ")"
+      LocalError(n.info, errGenerated, err)
       return newOrPrevType(tyError, prev, c)
 
     var isConcrete = true

compiler/sigmatch.nim

@@ -180,15 +180,9 @@ proc argTypeToString(arg: PNode): string =
   else:
     result = arg.typ.typeToString
 
-proc NotFoundError*(c: PContext, n: PNode) =
-  # Gives a detailed error message; this is separated from semOverloadedCall,
-  # as semOverlodedCall is already pretty slow (and we need this information
-  # only in case of an error).
-  if c.InCompilesContext > 0: 
-    # fail fast:
-    GlobalError(n.info, errTypeMismatch, "")
-  var result = msgKindToString(errTypeMismatch)
-  for i in countup(1, sonsLen(n) - 1):
+proc describeArgs*(c: PContext, n: PNode, startIdx = 1): 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]))
@@ -204,6 +198,16 @@ proc NotFoundError*(c: PContext, n: PNode) =
     if arg.typ.kind == tyError: return
     add(result, argTypeToString(arg))
     if i != sonsLen(n) - 1: add(result, ", ")
+
+proc NotFoundError*(c: PContext, n: PNode) =
+  # Gives a detailed error message; this is separated from semOverloadedCall,
+  # as semOverlodedCall is already pretty slow (and we need this information
+  # only in case of an error).
+  if c.InCompilesContext > 0: 
+    # fail fast:
+    GlobalError(n.info, errTypeMismatch, "")
+  var result = msgKindToString(errTypeMismatch)
+  add(result, describeArgs(c, n))
   add(result, ')')
   var candidates = ""
   var o: TOverloadIter

tests/compile/tgeneric4.nim

@@ -0,0 +1,10 @@
+type
+  TIDGen*[A: Ordinal] = object
+    next: A
+    free: seq[A]
+
+proc newIDGen*[A]: TIDGen[A] =
+    newSeq result.free, 0
+
+var x = newIDGen[int]()
+