go/types: if base type for an alias is known, use it when needed

Because methods are type-checked before the receiver base type
is "complete" (i.e., they are checked as part of the receiver
base type), situations occur where aliases of those base types
are used (in those methods) but the alias types are not known
yet (even though their base types are known).

This fix is a temporary work-around that looks syntactically
for the base types of alias types and uses those base types
when we refer to an "incomplete" alias type. The work-around
is completely localized and guarded with a flag so it can be
disabled at short notice.

The correct fix (slated for 1.12) is to decouple type-checking
of methods from their receiver base types. See issue #26854.

Fixes #26390.

Change-Id: I66cc9d834b220c254ac00e671a137cf8a3da59c1
Reviewed-on: https://go-review.googlesource.com/128435
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/check_test.go

@@ -91,6 +91,7 @@ var tests = [][]string{
 	{"testdata/issues.src"},
 	{"testdata/blank.src"},
 	{"testdata/issue25008b.src", "testdata/issue25008a.src"}, // order (b before a) is crucial!
+	{"testdata/issue26390.src"},                              // stand-alone test to ensure case is triggered
 }
 
 var fset = token.NewFileSet()

src/go/types/decl.go

@@ -165,6 +165,31 @@ func (check *Checker) objDecl(obj Object, def *Named, path []*TypeName) {
 			}
 
 		case *TypeName:
+			// fixFor26390 enables a temporary work-around to handle alias type names
+			// that have not been given a type yet even though the underlying type
+			// is already known. See testdata/issue26390.src for a simple example.
+			// Set this flag to false to disable this code quickly (and comment
+			// out the new test in decls4.src that will fail again).
+			// TODO(gri) remove this for Go 1.12 in favor of a more comprehensive fix
+			const fixFor26390 = true
+			if fixFor26390 {
+				// If we have a package-level alias type name that has not been
+				// given a type yet but the underlying type is a type name that
+				// has been given a type already, don't report a cycle but use
+				// the underlying type name's type instead. The cycle shouldn't
+				// exist in the first place in this case and is due to the way
+				// methods are type-checked at the moment. See also the comment
+				// at the end of Checker.typeDecl below.
+				if d := check.objMap[obj]; d != nil && d.alias && obj.typ == Typ[Invalid] {
+					// If we can find the underlying type name syntactically
+					// and it has a type, use that type.
+					if tname := check.resolveBaseTypeName(ast.NewIdent(obj.name)); tname != nil && tname.typ != nil {
+						obj.typ = tname.typ
+						break
+					}
+				}
+			}
+
 			if useCycleMarking && check.typeCycle(obj) {
 				// break cycle
 				// (without this, calling underlying()

src/go/types/testdata/issue26390.src

@@ -0,0 +1,11 @@
+// Copyright 2018 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 issue26390
+
+type A = T
+
+func (t *T) m() *A { return t }
+
+type T struct{}