cmd/compile: fix crash on bare package name in constant declarations

Fixes #11361.

Change-Id: I70b8808f97f0e07de680e7e6ede1322ea0fdbbc0
Reviewed-on: https://go-review.googlesource.com/21936Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -540,8 +540,15 @@ func treecopy(n *Node, lineno int32) *Node {
 		}
 		return n
 
+	case OPACK:
+		// OPACK nodes are never valid in const value declarations,
+		// but allow them like any other declared symbol to avoid
+		// crashing (golang.org/issue/11361).
+		fallthrough
+
 	case ONAME, OLITERAL, OTYPE:
 		return n
+
 	}
 }
 

test/fixedbugs/issue11361.go

+// errorcheck
+
+// Copyright 2016 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 a
+
+import "fmt"  // ERROR "imported and not used"
+
+const n = fmt // ERROR "fmt without selector" "fmt is not a constant"