go/types, types2: add missing Named.unpack call in Checker.hasVarSize

CL 734980 swapped use of Type.Underlying() in Checker.hasVarSize for
traversal of Named.fromRHS. It forgot to call Named.unpack() before
inspecting Named.fromRHS, allowing access of unexpanded instantiated
types.

These unexpanded instantiated types are then mistakenly marked as having
fixed size, which fails assertions downstream. This change adds the
missing Named.unpack() call and swaps direct access of Named.fromRHS for
Named.rhs(), which verifies such access in debug mode.

Fixes #77382

Change-Id: I324bbbbf790f8b09e95902ebe67f775483f88417
Reviewed-on: https://go-review.googlesource.com/c/go/+/740620
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/types2/builtins.go

@@ -1009,7 +1009,8 @@ func sliceElem(x *operand) (Type, *typeError) {
 // yet been checked.
 func (check *Checker) hasVarSize(t Type) bool {
 	// Note: We could use Underlying here, but passing through the RHS may yield
-	// better error messages.
+	// better error messages and allows us to stash the result on each traversed
+	// Named type.
 	switch t := Unalias(t).(type) {
 	case *Named:
 		if t.stateHas(hasVarSize) {
@@ -1025,7 +1026,9 @@ func (check *Checker) hasVarSize(t Type) bool {
 		check.push(t.obj)
 		defer check.pop()
 
-		varSize := check.hasVarSize(t.fromRHS)
+		// Careful, we're inspecting t.fromRHS, so we need to unpack first.
+		t.unpack()
+		varSize := check.hasVarSize(t.rhs())
 
 		t.mu.Lock()
 		defer t.mu.Unlock()

src/go/types/builtins.go

@@ -1012,7 +1012,8 @@ func sliceElem(x *operand) (Type, *typeError) {
 // yet been checked.
 func (check *Checker) hasVarSize(t Type) bool {
 	// Note: We could use Underlying here, but passing through the RHS may yield
-	// better error messages.
+	// better error messages and allows us to stash the result on each traversed
+	// Named type.
 	switch t := Unalias(t).(type) {
 	case *Named:
 		if t.stateHas(hasVarSize) {
@@ -1028,7 +1029,9 @@ func (check *Checker) hasVarSize(t Type) bool {
 		check.push(t.obj)
 		defer check.pop()
 
-		varSize := check.hasVarSize(t.fromRHS)
+		// Careful, we're inspecting t.fromRHS, so we need to unpack first.
+		t.unpack()
+		varSize := check.hasVarSize(t.rhs())
 
 		t.mu.Lock()
 		defer t.mu.Unlock()

src/internal/types/testdata/fixedbugs/issue77382.go

@@ -0,0 +1,15 @@
+// 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 p
+
+import "unsafe"
+
+type T1[T any] [42]T2[T]
+
+type T2[T any] [42]T
+
+func _[T any]() {
+	_ = unsafe.Sizeof(T1[T]{})
+}