cmd/compile: enable promoted field selectors as keys in struct literals

Switch the generated UIR version from V2 to V3.
Adjust cmd/compile/internal/types to accept promoted field selectors
in composite literals.

Fixes #9859.

Change-Id: Ie314e28567cfa6cf4c9e962a07b32dd05b06bf5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/755740
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/noder/unified.go

@@ -25,8 +25,8 @@ import (
 )
 
 // uirVersion is the unified IR version to use for encoding/decoding.
-// Use V2 as the encoded version for aliastypeparams.
-const uirVersion = pkgbits.V2
+// Use V3 for promoted struct field support in composite literals.
+const uirVersion = pkgbits.V3
 
 // localPkgReader holds the package reader used for reading the local
 // package. It exists so the unified IR linker can refer back to it

src/cmd/compile/internal/typecheck/expr.go

@@ -312,6 +312,19 @@ func tcStructLitKey(typ *types.Type, kv *ir.KeyExpr) *ir.StructKeyExpr {
 		return ir.NewStructKeyExpr(kv.Pos(), f, kv.Value)
 	}
 
+	var f *types.Field
+	if p, ambig := dotpath(sym, typ, &f, false); p != nil {
+		if ambig {
+			base.Errorf("ambiguous promoted field '%v' in struct literal of type %v", sym, typ)
+			return nil
+		}
+		if f.IsMethod() {
+			base.Errorf("cannot use method '%v' in struct literal of type %v", sym, typ)
+			return nil
+		}
+		return ir.NewStructKeyExpr(kv.Pos(), f, kv.Value)
+	}
+
 	if ci := Lookdot1(nil, sym, typ, typ.Fields(), 2); ci != nil { // Case-insensitive lookup.
 		if visible(ci.Sym) {
 			base.Errorf("unknown field '%v' in struct literal of type %v (but does have %v)", sym, typ, ci.Sym)
@@ -323,7 +336,6 @@ func tcStructLitKey(typ *types.Type, kv *ir.KeyExpr) *ir.StructKeyExpr {
 		return nil
 	}
 
-	var f *types.Field
 	p, _ := dotpath(sym, typ, &f, true)
 	if p == nil || f.IsMethod() {
 		base.Errorf("unknown field '%v' in struct literal of type %v", sym, typ)
@@ -335,8 +347,8 @@ func tcStructLitKey(typ *types.Type, kv *ir.KeyExpr) *ir.StructKeyExpr {
 	for ei := len(p) - 1; ei >= 0; ei-- {
 		ep = append(ep, p[ei].field.Sym.Name)
 	}
-	ep = append(ep, sym.Name)
-	base.Errorf("cannot use promoted field %v in struct literal of type %v", strings.Join(ep, "."), typ)
+	ep = append(ep, f.Sym.Name)
+	base.Errorf("unknown field '%v' in struct literal of type %v (but does have %v)", sym, typ, strings.Join(ep, "."))
 	return nil
 }
 

src/cmd/compile/internal/walk/complit.go

@@ -226,7 +226,7 @@ func fixedlit(ctxt initContext, kind initKind, n *ir.CompLitExpr, var_ ir.Node,
 				return ir.BlankNode, r.Value
 			}
 			ir.SetPos(r)
-			return ir.NewSelectorExpr(base.Pos, ir.ODOT, var_, r.Sym()), r.Value
+			return ir.NewSelectorExpr(base.Pos, ir.OXDOT, var_, r.Sym()), r.Value
 		}
 	default:
 		base.Fatalf("fixedlit bad op: %v", n.Op())

test/complit2.go

@@ -0,0 +1,37 @@
+// 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.
+
+// Verify that composite literals using selectors for
+// embedded fields are assembled correctly.
+
+package main
+
+import "fmt"
+
+type A struct {
+	a int
+	B
+}
+
+type B struct {
+	b string
+	C
+}
+
+type C struct {
+	c any
+}
+
+func main() {
+	eq(A{1, B{b: "foo"}}, A{a: 1, b: "foo"})
+	eq(A{B: B{C: C{c: "foo"}}}, A{c: "foo"})
+}
+
+func eq(x, y any) {
+	if x != y {
+		panic(fmt.Sprintf("%v != %v", x, y))
+	}
+}