go/types: re-enable suppressed gcimporter tests

Port CL 577616 from types2 to go/types allows us
to re-enable these tests.

Fixes #66859.
Fixes #67436.
Updates #50259.
Updates #65294.

Change-Id: I573e51cf16546ac1c115beac5322b51dd998c881
Reviewed-on: https://go-review.googlesource.com/c/go/+/586236
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/internal/gcimporter/gcimporter_test.go

@@ -7,7 +7,6 @@ package gcimporter_test
 import (
 	"bytes"
 	"fmt"
-	"internal/godebug"
 	"internal/testenv"
 	"os"
 	"os/exec"
@@ -205,16 +204,6 @@ func TestImportTypeparamTests(t *testing.T) {
 				want := types.ObjectString(checkedObj, types.RelativeTo(checked))
 				want = sanitizeObjectString(want)
 
-				// TODO(golang/go#66859): investigate and reenable these tests,
-				// which fail with gotypesalias=1, soon to be the default.
-				if godebug.New("gotypesalias").Value() != "0" {
-					symbol := name + " in " + filepath.Base(filename)
-					switch symbol {
-					case "Eint2 in struct.go", "A in issue50259.go":
-						t.Skipf("%s requires gotypesalias=1", symbol)
-					}
-				}
-
 				if got != want {
 					t.Errorf("imported %q as %q, want %q", name, got, want)
 				}

src/go/types/resolver.go

@@ -665,8 +665,23 @@ func (check *Checker) packageObjects() {
 		}
 	}
 
-	if check.conf._EnableAlias {
+	if false && check.conf._EnableAlias {
 		// With Alias nodes we can process declarations in any order.
+		//
+		// TODO(adonovan): unfortunately, Alias nodes
+		// (GODEBUG=gotypesalias=1) don't entirely resolve
+		// problems with cycles. For example, in
+		// GOROOT/test/typeparam/issue50259.go,
+		//
+		// 	type T[_ any] struct{}
+		// 	type A T[B]
+		// 	type B = T[A]
+		//
+		// TypeName A has Type Named during checking, but by
+		// the time the unified export data is written out,
+		// its Type is Invalid.
+		//
+		// Investigate and reenable this branch.
 		for _, obj := range objList {
 			check.objDecl(obj, nil)
 		}