types2, go/types: add missing alias test to types2, simplify go/types test

Change-Id: I4d3377e704d718a1914135a444b9df59f84af179
Reviewed-on: https://go-review.googlesource.com/c/go/+/780120
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -0,0 +1,61 @@
+// 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 types2_test
+
+import (
+	"cmd/compile/internal/types2"
+	"testing"
+)
+
+func TestIssue74181(t *testing.T) {
+	src := `package p
+
+type AB = A[B]
+
+type _ struct {
+	_ AB
+}
+
+type B struct {
+	f *AB
+}
+
+type A[T any] struct{}
+`
+
+	pkg := mustTypecheck(src, nil, nil)
+	b := pkg.Scope().Lookup("B").Type()
+	if n, ok := b.(*types2.Named); ok {
+		if s, ok := n.Underlying().(*types2.Struct); ok {
+			got := s.Field(0).Type()
+			want := types2.NewPointer(pkg.Scope().Lookup("AB").Type())
+			if !types2.Identical(got, want) {
+				t.Errorf("wrong type for f: got %v, want %v", got, want)
+			}
+			return
+		}
+	}
+	t.Errorf("unexpected type for B: %v", b)
+}
+
+func TestPartialTypeCheckUndeclaredAliasPanic(t *testing.T) {
+	src := `package p
+
+type A = B // undeclared
+`
+
+	pkg, _ := typecheck(src, nil, nil) // don't panic on error
+	a := pkg.Scope().Lookup("A").Type()
+	if alias, ok := a.(*types2.Alias); ok {
+		got := alias.Rhs()
+		want := types2.Typ[types2.Invalid]
+
+		if !types2.Identical(got, want) {
+			t.Errorf("wrong type for B: got %v, want %v", got, want)
+		}
+		return
+	}
+	t.Errorf("unexpected type for A: %v", a)
+}

src/go/types/alias_test.go

@@ -5,8 +5,6 @@
 package types_test
 
 import (
-	"go/ast"
-	"go/token"
 	"go/types"
 	"testing"
 )
@@ -24,17 +22,10 @@ type B struct {
 	f *AB
 }
 
-type A[T any] struct{}`
-
-	fset := token.NewFileSet()
-	file := mustParse(fset, src)
-
-	conf := types.Config{}
-	pkg, err := conf.Check(file.Name.Name, fset, []*ast.File{file}, &types.Info{})
-	if err != nil {
-		t.Fatalf("could not type check: %v", err)
-	}
+type A[T any] struct{}
+`
 
+	pkg := mustTypecheck(src, nil, nil)
 	b := pkg.Scope().Lookup("B").Type()
 	if n, ok := b.(*types.Named); ok {
 		if s, ok := n.Underlying().(*types.Struct); ok {
@@ -52,15 +43,11 @@ type A[T any] struct{}`
 func TestPartialTypeCheckUndeclaredAliasPanic(t *testing.T) {
 	src := `package p
 
-type A = B // undeclared`
+type A = B // undeclared
+`
 
-	fset := token.NewFileSet()
-	file := mustParse(fset, src)
-
-	conf := types.Config{} // no error handler, panic
-	pkg, _ := conf.Check(file.Name.Name, fset, []*ast.File{file}, &types.Info{})
+	pkg, _ := typecheck(src, nil, nil) // don't panic on error
 	a := pkg.Scope().Lookup("A").Type()
-
 	if alias, ok := a.(*types.Alias); ok {
 		got := alias.Rhs()
 		want := types.Typ[types.Invalid]