cmd/compile: suppress callsite signatures if any type is unknown

Fixes #19012.

Fallback to return signatures without detailed types.
These error message will be of the form of issue:
* https://golang.org/issues/4215
* https://golang.org/issues/6750

So:
func f(x int, y uint) {
    return x > y
}

f(10, "a" < 3)

will give errors:
too many errors to return
too many arguments in call to f

instead of:

too many errors to return
  have (<T>)
  want ()
too many arguments in call to f
  have (number, <T>)
  want (number, number)

Change-Id: I680abc7cdd8444400e234caddf3ff49c2d69f53d
Reviewed-on: https://go-review.googlesource.com/36806Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/typecheck.go

@@ -2700,17 +2700,18 @@ out:
 
 notenough:
 	if n == nil || !n.Diag {
+		details := errorDetails(nl, tstruct, isddd)
 		if call != nil {
 			// call is the expression being called, not the overall call.
 			// Method expressions have the form T.M, and the compiler has
 			// rewritten those to ONAME nodes but left T in Left.
 			if call.Op == ONAME && call.Left != nil && call.Left.Op == OTYPE {
-				yyerror("not enough arguments in call to method expression %v\n\thave %s\n\twant %v", call, nl.retsigerr(isddd), tstruct)
+				yyerror("not enough arguments in call to method expression %v%s", call, details)
 			} else {
-				yyerror("not enough arguments in call to %v\n\thave %s\n\twant %v", call, nl.retsigerr(isddd), tstruct)
+				yyerror("not enough arguments in call to %v%s", call, details)
 			}
 		} else {
-			yyerror("not enough arguments to %v\n\thave %s\n\twant %v", op, nl.retsigerr(isddd), tstruct)
+			yyerror("not enough arguments to %v%s", op, details)
 		}
 		if n != nil {
 			n.Diag = true
@@ -2720,14 +2721,30 @@ notenough:
 	goto out
 
 toomany:
+	details := errorDetails(nl, tstruct, isddd)
 	if call != nil {
-		yyerror("too many arguments in call to %v\n\thave %s\n\twant %v", call, nl.retsigerr(isddd), tstruct)
+		yyerror("too many arguments in call to %v%s", call, details)
 	} else {
-		yyerror("too many arguments to %v\n\thave %s\n\twant %v", op, nl.retsigerr(isddd), tstruct)
+		yyerror("too many arguments to %v%s", op, details)
 	}
 	goto out
 }
 
+func errorDetails(nl Nodes, tstruct *Type, isddd bool) string {
+	// If we don't know any type at a call site, let's suppress any return
+	// message signatures. See Issue https://golang.org/issues/19012.
+	if tstruct == nil {
+		return ""
+	}
+	// If any node has an unknown type, suppress it as well
+	for _, n := range nl.Slice() {
+		if n.Type == nil {
+			return ""
+		}
+	}
+	return fmt.Sprintf("\n\thave %s\n\twant %v", nl.retsigerr(isddd), tstruct)
+}
+
 // sigrepr is a type's representation to the outside world,
 // in string representations of return signatures
 // e.g in error messages about wrong arguments to return.

test/fixedbugs/issue19012.go

+// errorcheck
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 19012: if we have any unknown type at a call site,
+// we must ensure that we return to the user a suppressed
+// error message saying instead of including <T> in
+// the message.
+
+package main
+
+func f(x int, y uint) {
+	if true {
+		return "a" > 10 // ERROR "^too many arguments to return$" "."
+	}
+	return "gopher" == true, 10 // ERROR "^too many arguments to return$" "."
+}
+
+func main() {
+	f(2, 3 < "x", 10) // ERROR "^too many arguments in call to f$" "."
+
+	f(10, 10, "a") // ERROR "too many arguments in call to f\n\thave \(number, number, string\)\n\twant \(int, uint\)"
+}