runtime: panic with the right error on iface conversion

A straight conversion from a type T to an interface type I, where T does
not implement I, should always panic with an interface conversion error
that shows the missing method.  This was not happening if the conversion
was done once using the comma-ok form (the result would not be OK) and
then again in a straight conversion.  Due to an error in the runtime
package the second conversion was failing with a nil pointer
dereference.

Fixes #16130.

Change-Id: I8b9fca0f1bb635a6181b8b76de8c2385bb7ac2d2
Reviewed-on: https://go-review.googlesource.com/24284
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michel Lespinasse <walken@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/iface.go

@@ -54,7 +54,6 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 		for m = (*itab)(atomic.Loadp(unsafe.Pointer(&hash[h]))); m != nil; m = m.link {
 			if m.inter == inter && m._type == typ {
 				if m.bad != 0 {
-					m = nil
 					if !canfail {
 						// this can only happen if the conversion
 						// was already done once using the , ok form
@@ -64,6 +63,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 						// adding the itab again, which will throw an error.
 						additab(m, locked != 0, false)
 					}
+					m = nil
 				}
 				if locked != 0 {
 					unlock(&ifaceLock)

test/fixedbugs/issue16130.go

+// run
+
+// Copyright 2016 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.
+
+// Test that an interface conversion error panics with an "interface
+// conversion" run-time error. It was (incorrectly) panicing with a
+// "nil pointer dereference."
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+type I interface {
+	Get() int
+}
+
+func main() {
+	defer func() {
+		r := recover()
+		if r == nil {
+			panic("expected panic")
+		}
+		re, ok := r.(runtime.Error)
+		if !ok {
+			panic(fmt.Sprintf("got %T, expected runtime.Error", r))
+		}
+		if !strings.Contains(re.Error(), "interface conversion") {
+			panic(fmt.Sprintf("got %q, expected interface conversion error", re.Error()))
+		}
+	}()
+	e := (interface{})(0)
+	if _, ok := e.(I); ok {
+		panic("unexpected interface conversion success")
+	}
+	fmt.Println(e.(I))
+	panic("unexpected interface conversion success")
+}