cmd/gc: do not display ~b identifiers in error messages

Instead of errors like:

./blank2.go:15: cannot use ~b1 (type []int) as type int in assignment

we now have:

./blank2.go:15: cannot use _ (type []int) as type int in assignment

Less confusing for users.

Fixes #9521

Change-Id: Ieab9859040e8e0df95deeaee7eeb408d3be61c0f
Reviewed-on: https://go-review.googlesource.com/9902Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/gc/fmt.go

@@ -1127,7 +1127,7 @@ func exprfmt(n *Node, prec int) string {
 		// Special case: name used as local variable in export.
 	// _ becomes ~b%d internally; print as _ for export
 	case ONAME:
-		if fmtmode == FExp && n.Sym != nil && n.Sym.Name[0] == '~' && n.Sym.Name[1] == 'b' {
+		if (fmtmode == FExp || fmtmode == FErr) && n.Sym != nil && n.Sym.Name[0] == '~' && n.Sym.Name[1] == 'b' {
 			return "_"
 		}
 		if fmtmode == FExp && n.Sym != nil && !isblank(n) && n.Vargen > 0 {

test/fixedbugs/issue9521.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Test that an incorrect use of the blank identifer is caught.
+// Does not compile.
+
+package main
+
+func f() (_, _ []int) { return }
+
+func main() {
+	_ = append(f()) // ERROR "cannot use _"
+}