runtime: slightly better error message for assertion panics with identical looking types

Fixes #18911.

Change-Id: Ice10f37460a4f0a66cddeacfe26c28045f5e60fe
Reviewed-on: https://go-review.googlesource.com/116255
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/error.go

@@ -36,8 +36,13 @@ func (e *TypeAssertionError) Error() string {
 		return "interface conversion: " + inter + " is nil, not " + e.assertedString
 	}
 	if e.missingMethod == "" {
-		return "interface conversion: " + inter + " is " + e.concreteString +
+		msg := "interface conversion: " + inter + " is " + e.concreteString +
 			", not " + e.assertedString
+		if e.concreteString == e.assertedString {
+			// provide slightly clearer error message
+			msg += " (types from different packages)"
+		}
+		return msg
 	}
 	return "interface conversion: " + e.concreteString + " is not " + e.assertedString +
 		": missing method " + e.missingMethod

test/fixedbugs/issue18911.dir/a.go

@@ -0,0 +1,7 @@
+// Copyright 2018 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.
+
+package a
+
+var X interface{} = struct{ x int }{}

test/fixedbugs/issue18911.dir/b.go

@@ -0,0 +1,21 @@
+// Copyright 2018 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.
+
+package main
+
+import "./a"
+import "strings"
+
+func main() {
+	defer func() {
+		p, ok := recover().(error)
+		if ok && strings.Contains(p.Error(), "different packages") {
+			return
+		}
+		panic(p)
+	}()
+
+	// expected to fail and report two identical looking (but different) types
+	_ = a.X.(struct{ x int })
+}

test/fixedbugs/issue18911.go

@@ -0,0 +1,7 @@
+// rundir
+
+// Copyright 2018 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.
+
+package ignore