go/types: minimal support for alias declarations: don't crash

For #16339

Change-Id: I8927f40e0fd166795f41c784ad92449743f73af5
Reviewed-on: https://go-review.googlesource.com/30213Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/check_test.go

@@ -72,6 +72,7 @@ var tests = [][]string{
 	{"testdata/const1.src"},
 	{"testdata/constdecl.src"},
 	{"testdata/vardecl.src"},
+	{"testdata/aliasdecl.src"},
 	{"testdata/expr0.src"},
 	{"testdata/expr1.src"},
 	{"testdata/expr2.src"},

src/go/types/resolver.go

@@ -274,6 +274,9 @@ func (check *Checker) collectObjects() {
 							check.declare(fileScope, nil, obj, token.NoPos)
 						}
 
+					case *ast.AliasSpec:
+						check.errorf(s.Name.Pos(), "cannot handle alias declarations yet")
+
 					case *ast.ValueSpec:
 						switch d.Tok {
 						case token.CONST:

src/go/types/testdata/aliasdecl.src

+// 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 aliasdecl
+
+import "math"
+
+const _ = math.Pi
+const c /* ERROR "cannot handle alias declarations yet" */ => math.Pi