cmd/compile: suppress duplicate type errors

If we've already complained about a type T,
don't complain again about further expressions
involving it.

Fixes #20245 and hopefully all of its ilk.

Change-Id: Ic0abe8235d52e8a7ac40e3615aea8f3a54fd7cec
Reviewed-on: https://go-review.googlesource.com/42690
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -2156,7 +2156,9 @@ OpSwitch:
 
 	evconst(n)
 	if n.Op == OTYPE && top&Etype == 0 {
+		if !n.Type.Broke() {
 			yyerror("type %v is not an expression", n.Type)
+		}
 		n.Type = nil
 		return n
 	}

test/fixedbugs/issue11614.go

@@ -15,7 +15,7 @@ type I interface {
 }
 
 func n() {
-	(I) // ERROR "type I is not an expression"
+	(I)
 }
 
 func m() {

test/fixedbugs/issue20233.go

@@ -8,4 +8,4 @@
 
 package p
 
-var f = func(...A) // ERROR "type func(....*) is not an expression" ERROR "undefined: A"
+var f = func(...A) // ERROR "undefined: A"

test/fixedbugs/issue20245.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 20245: panic while formatting an error message
+
+package p
+
+var e = interface{ I1 } // ERROR "undefined: I1"