reflect: use &zeroVal[0] instead of nil for data field for zero-sized payloads

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

Fixes #77779

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>

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,

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()
+}