From d62c6c3c39ddc59e3003304f98661598b5fd80de Mon Sep 17 00:00:00 2001
From: Josh Bleecher Snyder <josharian@gmail.com>
Date: Thu, 4 May 2017 11:43:41 -0700
Subject: [PATCH] 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 |  4 +++-
 test/fixedbugs/issue11614.go             |  2 +-
 test/fixedbugs/issue20233.go             |  2 +-
 test/fixedbugs/issue20245.go             | 11 +++++++++++
 4 files changed, 16 insertions(+), 3 deletions(-)
 create mode 100644 test/fixedbugs/issue20245.go

diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index e66ce5dc985..6f544a5b9ae 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/src/cmd/compile/internal/gc/typecheck.go
@@ -2156,7 +2156,9 @@ OpSwitch:
 
 	evconst(n)
 	if n.Op == OTYPE && top&Etype == 0 {
-		yyerror("type %v is not an expression", n.Type)
+		if !n.Type.Broke() {
+			yyerror("type %v is not an expression", n.Type)
+		}
 		n.Type = nil
 		return n
 	}
diff --git a/test/fixedbugs/issue11614.go b/test/fixedbugs/issue11614.go
index 959643a5147..91f134d44a9 100644
--- a/test/fixedbugs/issue11614.go
+++ b/test/fixedbugs/issue11614.go
@@ -15,7 +15,7 @@ type I interface {
 }
 
 func n() {
-	(I) // ERROR "type I is not an expression"
+	(I)
 }
 
 func m() {
diff --git a/test/fixedbugs/issue20233.go b/test/fixedbugs/issue20233.go
index 5734cf44ef0..4dec4e458be 100644
--- a/test/fixedbugs/issue20233.go
+++ b/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"
diff --git a/test/fixedbugs/issue20245.go b/test/fixedbugs/issue20245.go
new file mode 100644
index 00000000000..b07dbe20de6
--- /dev/null
+++ b/test/fixedbugs/issue20245.go
@@ -0,0 +1,11 @@
+// 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"