go/types: avoid certain problems with recursive alias type declarations

It is possible to create certain recursive type declarations involving
alias types which cause the type-checker to produce an (invalid) type
for the alias because it is not yet available. By type-checking alias
declarations in a 2nd phase, the problem is mitigated a bit since it
requires more convoluted alias declarations for the problem to appear.

Also re-enable testing of fixedbugs/issue27232.go again (which was the
original cause for this change).

Updates #28576.

Change-Id: If6f9656a95262e6575b01c4a003094d41551564b
Reviewed-on: https://go-review.googlesource.com/c/147597Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/resolver.go

@@ -570,7 +570,25 @@ func (check *Checker) packageObjects() {
 		}
 	}
 
+	// We process non-alias declarations first, in order to avoid situations where
+	// the type of an alias declaration is needed before it is available. In general
+	// this is still not enough, as it is possible to create sufficiently convoluted
+	// recursive type definitions that will cause a type alias to be needed before it
+	// is available (see issue #25838 for examples).
+	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	var aliasList []*TypeName
+	// phase 1
 	for _, obj := range objList {
+		// If we have a type alias, collect it for the 2nd phase.
+		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].alias {
+			aliasList = append(aliasList, tname)
+			continue
+		}
+
+		check.objDecl(obj, nil)
+	}
+	// phase 2
+	for _, obj := range aliasList {
 		check.objDecl(obj, nil)
 	}
 

src/go/types/stdlib_test.go

@@ -180,7 +180,6 @@ func TestStdFixed(t *testing.T) {
 		"issue22200b.go", // go/types does not have constraints on stack size
 		"issue25507.go",  // go/types does not have constraints on stack size
 		"issue20780.go",  // go/types does not have constraints on stack size
-		"issue27232.go",  // go/types has a bug with alias type (issue #28576)
 	)
 }