reflect: handle zero-sized fields of directly-stored structures correctly

type W struct {
	E struct{}
	X *byte
}

type W is a "direct" type. That is, it is a pointer-ish type that can
be stored directly as the second word of an interface.

But if we ask reflect for W's first field, that value must *not* be
direct, as zero-sized things cannot be stored directly.

This was a problem introduced in CL 681937. Before that, types like W
were not eligible for directness.

Fixes #74935

Change-Id: Idefb55c23eaa59153009f863bad611593981e5cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/694195
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: randall77

src/reflect/value.go

@@ -1277,6 +1277,17 @@ func (v Value) Field(i int) Value {
 			fl |= flagStickyRO
 		}
 	}
+	if fl&flagIndir == 0 && typ.Size() == 0 {
+		// Special case for picking a field out of a direct struct.
+		// A direct struct must have a pointer field and possibly a
+		// 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 can avoid pinning a possibly now-unused object.
+		return Value{typ, nil, fl | flagIndir}
+	}
+
 	// Either flagIndir is set and v.ptr points at struct,
 	// or flagIndir is not set and v.ptr is the actual struct data.
 	// In the former case, we want v.ptr + offset.

test/fixedbugs/issue74935.go

@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2025 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 W struct {
+	E struct{}
+	X *byte
+}
+
+func main() {
+	w := reflect.ValueOf(W{})
+	_ = w.Field(0).Interface()
+}