gc: omit argument names from function types in error messages

Fixes #2563

R=rsc
CC=golang-dev
https://golang.org/cl/5495047

src/cmd/gc/fmt.c

@@ -680,7 +680,7 @@ typefmt(Fmt *fp, Type *t)
 	case TSTRUCT:
 		if(t->funarg) {
 			fmtstrcpy(fp, "(");
-			if(fmtmode == FTypeId) {	// no argument names on function signature, and no "noescape" tags
+			if(fmtmode == FTypeId || fmtmode == FErr) {	// no argument names on function signature, and no "noescape" tags
 				for(t1=t->type; t1!=T; t1=t1->down)
 					if(t1->down)
 						fmtprint(fp, "%hT, ", t1);

test/fixedbugs/bug389.go

+// errchk $G $D/$F.go
+
+// Copyright 2011 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 2563
+package foo
+
+func fn(a float32) {}
+
+var f func(arg int) = fn  // ERROR "cannot use fn .type func.float32.. as type func.int. in assignment"
\ No newline at end of file