[dev.typealias] cmd/compile: avoid false positive cycles from type aliases

For #18130.
Fixes #18640.

Change-Id: I26cf1d1b78cca6ef207cc4333f30a9011ef347c9
Reviewed-on: https://go-review.googlesource.com/35831
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -339,13 +339,15 @@ func Main() {
 	// Phase 1: const, type, and names and types of funcs.
 	//   This will gather all the information about types
 	//   and methods but doesn't depend on any of it.
+	//   We also defer type alias declarations until phase 2
+	//   to avoid cycles like #18640.
 	defercheckwidth()
 
 	// Don't use range--typecheck can add closures to xtop.
 	timings.Start("fe", "typecheck", "top1")
 	for i := 0; i < len(xtop); i++ {
 		n := xtop[i]
-		if op := n.Op; op != ODCL && op != OAS && op != OAS2 {
+		if op := n.Op; op != ODCL && op != OAS && op != OAS2 && (op != ODCLTYPE || !n.Left.Name.Param.Alias) {
 			xtop[i] = typecheck(n, Etop)
 		}
 	}
@@ -357,7 +359,7 @@ func Main() {
 	timings.Start("fe", "typecheck", "top2")
 	for i := 0; i < len(xtop); i++ {
 		n := xtop[i]
-		if op := n.Op; op == ODCL || op == OAS || op == OAS2 {
+		if op := n.Op; op == ODCL || op == OAS || op == OAS2 || op == ODCLTYPE && n.Left.Name.Param.Alias {
 			xtop[i] = typecheck(n, Etop)
 		}
 	}

test/fixedbugs/issue18640.go

@@ -0,0 +1,26 @@
+// compile
+
+// Copyright 2017 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 p
+
+type (
+	a = b
+	b struct {
+		*a
+	}
+
+	c struct {
+		*d
+	}
+	d = c
+
+	e = f
+	f = g
+	g = []h
+	h i
+	i = j
+	j = e
+)