[release-branch.go1.23] cmd/compile/internal/importer: enable aliases

Flips the pkgReader.enableAlias flag to true when reading unified IR.
This was disabled while resolving golang#66873. This resolves the TODO to
flip it back to true.

Fixes golang#70394
Fixes golang#70517
Updates golang#66873

Change-Id: Ifd52b0f9510d6bcf151de1c9a18d71ab548c14e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/604099
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit 209ed1a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/631855
Commit-Queue: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/importer/gcimporter_test.go

@@ -582,6 +582,23 @@ func TestIssue25596(t *testing.T) {
 	compileAndImportPkg(t, "issue25596")
 }
 
+func TestIssue70394(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	pkg := compileAndImportPkg(t, "alias")
+	obj := lookupObj(t, pkg.Scope(), "A")
+
+	typ := obj.Type()
+	if _, ok := typ.(*types2.Alias); !ok {
+		t.Fatalf("type of %s is %s, wanted an alias", obj, typ)
+	}
+}
+
 func importPkg(t *testing.T, path, srcDir string) *types2.Package {
 	pkg, err := Import(make(map[string]*types2.Package), path, srcDir, nil)
 	if err != nil {

src/cmd/compile/internal/importer/testdata/alias.go

@@ -0,0 +1,7 @@
+// Copyright 2024 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 testdata
+
+type A = int32

src/cmd/compile/internal/importer/ureader.go

@@ -29,11 +29,9 @@ func ReadPackage(ctxt *types2.Context, imports map[string]*types2.Package, input
 	pr := pkgReader{
 		PkgDecoder: input,
 
-		ctxt:    ctxt,
-		imports: imports,
-		// Currently, the compiler panics when using Alias types.
-		// TODO(gri) set to true once this is fixed (issue #66873)
-		enableAlias: false,
+		ctxt:        ctxt,
+		imports:     imports,
+		enableAlias: true,
 
 		posBases: make([]*syntax.PosBase, input.NumElems(pkgbits.RelocPosBase)),
 		pkgs:     make([]*types2.Package, input.NumElems(pkgbits.RelocPkg)),