From ff84d756df91f94289bef164765142632922a6bc Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Tue, 3 Dec 2013 17:51:54 -0800
Subject: [PATCH] go.tools/go/types: avoid spurious error in a common case

R=adonovan
CC=golang-dev
https://golang.org/cl/36900043
---
 go/types/expr.go             |  2 +-
 go/types/testdata/errors.src | 11 ++++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/go/types/expr.go b/go/types/expr.go
index 20f64b15..6dd67543 100644
--- a/go/types/expr.go
+++ b/go/types/expr.go
@@ -464,7 +464,7 @@ func (check *checker) updateExprType(x ast.Expr, typ Type, final bool) {
 
 // convertUntyped attempts to set the type of an untyped value to the target type.
 func (check *checker) convertUntyped(x *operand, target Type) {
-	if x.mode == invalid || isTyped(x.typ) {
+	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
 		return
 	}
 
diff --git a/go/types/testdata/errors.src b/go/types/testdata/errors.src
index 11380078..f8acc0c7 100644
--- a/go/types/testdata/errors.src
+++ b/go/types/testdata/errors.src
@@ -2,11 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// testing precise operand formatting in error messages
-// (matching messages are regular expressions, hence the \'s)
-
 package errors
 
+// Testing precise operand formatting in error messages
+// (matching messages are regular expressions, hence the \'s).
 func f(x int, m map[string]int) {
 	// no values
 	_ = f /* ERROR "f\(0, m\) \(no value\) used as value" */ (0, m)
@@ -44,3 +43,9 @@ func _() {
 	0 // ERROR "0 .* is not used"
 	0 // ERROR 0 .* is not used
 }
+
+// Don't report spurious errors as a consequence of earlier errors.
+// Add more tests as needed.
+func _() {
+	if err := foo /* ERROR undeclared */ (); err != nil /* no error here */ {}
+}
\ No newline at end of file