cmd/compile: avoid compiler crash for recursive interface type

This change is a simple work-around to avoid a compiler crash
and provide a reasonable error message. A future change should
fix the root cause for this problem.

Fixes #23823.

Change-Id: Ifc80d9f4d35e063c378e54d5cd8d1cf4c0d2ec6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/175518
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: griesemer

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

@@ -172,7 +172,16 @@ func dowidth(t *types.Type) {
 	if t.Width == -2 {
 		if !t.Broke() {
 			t.SetBroke(true)
-			yyerrorl(asNode(t.Nod).Pos, "invalid recursive type %v", t)
+			// t.Nod should not be nil here, but in some cases is appears to be
+			// (see issue #23823). For now (temporary work-around) at a minimum
+			// don't crash and provide a meaningful error message.
+			// TODO(gri) determine the correct fix during a regular devel cycle
+			// (see issue #31872).
+			if t.Nod == nil {
+				yyerror("invalid recursive type %v", t)
+			} else {
+				yyerrorl(asNode(t.Nod).Pos, "invalid recursive type %v", t)
+			}
 		}
 
 		t.Width = 0

test/fixedbugs/issue23823.go

@@ -1,19 +1,15 @@
-// compile
+// errorcheck
 
 // 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 p
 
-// The compiler cannot handle this. Disabled for now.
-// See issue #25838.
-/*
 type I1 = interface {
 	I2
 }
 
-type I2 interface {
+type I2 interface { // ERROR "invalid recursive type"
 	I1
 }
-*/