[release-branch.go1.26] reflect: use &zeroVal[0] instead of nil for data field for...

Because our wrapper functions barf if the pointer is nil, even if
we don't actually dereference the pointer.

Fixes #77780

Change-Id: Ib1b93d9f0fdc771cd884137007508ba2b1da4b7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/748660
Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
(cherry picked from commit 60dc96479a85db0e245a5c14d964628169abedf4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/748720
Reviewed-by: David Chase <drchase@google.com>

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 8c8acbaa9abdeb1ded8db01ef7d8cbbd5a9bf4c5..f192eb5974eed0f7547e7d8433be5ecebe7b64c7 100644 (file)
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1288,9 +1288,10 @@ func (v Value) Field(i int) Value {
                // bunch of zero-sized fields. We must return the zero-sized
                // fields indirectly, as only ptr-shaped things can be direct.
                // See issue 74935.
-               // We use nil instead of v.ptr as it doesn't matter and
+               // We use &zeroVal[0] instead of v.ptr as it doesn't matter and
                // we can avoid pinning a possibly now-unused object.
-               return Value{typ, nil, fl | flagIndir}
+               // Don't use nil, see issue 77779.
+               return Value{typ, unsafe.Pointer(&zeroVal[0]), fl | flagIndir}
        }
 
        // Either flagIndir is set and v.ptr points at struct,

diff --git a/test/fixedbugs/issue77779.go b/test/fixedbugs/issue77779.go
new file mode 100644 (file)
index 0000000..1a61b2c
--- /dev/null
+++ b/test/fixedbugs/issue77779.go
@@ -0,0 +1,45 @@
+// run
+
+// Copyright 2026 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 (
+       "reflect"
+)
+
+type Renderer interface {
+       Render() error
+}
+
+type ZeroSize struct{}
+
+func (ZeroSize) Render() error { return nil }
+
+type Data struct {
+       X, Y, Z int
+}
+
+// Container is pointer-sized (8 bytes): zero-size embed + one pointer field.
+// This triggers Go 1.26 interface inlining, which produces a nil data pointer
+// for the zero-size field when extracted via reflect.Value.Interface().
+type Container struct {
+       ZeroSize
+       Data *Data
+}
+
+func main() {
+       render(Container{})
+       render(&Container{})
+}
+
+func render(iface any) {
+       if reflect.ValueOf(iface).Kind() == reflect.Ptr {
+               _ = reflect.ValueOf(iface).Elem().Field(0).Interface().(Renderer).Render()
+               return
+       }
+
+       _ = reflect.ValueOf(iface).Field(0).Interface().(Renderer).Render()
+}