cmd/compile/internal/types2: revert to three-phase alias resolution

This change reenables the legacy three-phase resolution
(non-alias typenames, aliases, the rest) even when
GODEBUG=gotypesalias=1. Unfortunately the existing test case
for #50259 causes the simpler logic to fail.

Updates #50259
Updates #65294

Change-Id: Ibfaf8146e46760718673a916a9b220a9d678409a
Reviewed-on: https://go-review.googlesource.com/c/go/+/577616
Reviewed-by: Matthew Dempsky <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

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

@@ -677,8 +677,23 @@ func (check *Checker) packageObjects() {
 		}
 	}
 
-	if check.enableAlias {
+	if false && check.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)
 		}