cmd/cgo: support multiple-value special form in VarDecl

Fixes #13930.

Change-Id: I124b7d31d1f2be05b7f23dafd1e52d9f3f02f3f0
Reviewed-on: https://go-review.googlesource.com/18623
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/issue13930.go

+// 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.
+
+// Issue 13930.  Test that cgo's multiple-value special form for
+// C function calls works in variable declaration statements.
+
+package cgotest
+
+// #include <stdlib.h>
+import "C"
+
+var _, _ = C.abs(0)

src/cmd/cgo/ast.go

@@ -447,7 +447,11 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 	case *ast.ImportSpec:
 	case *ast.ValueSpec:
 		f.walk(&n.Type, "type", visit)
-		f.walk(n.Values, "expr", visit)
+		if len(n.Names) == 2 && len(n.Values) == 1 {
+			f.walk(&n.Values[0], "as2", visit)
+		} else {
+			f.walk(n.Values, "expr", visit)
+		}
 	case *ast.TypeSpec:
 		f.walk(&n.Type, "type", visit)
 

src/cmd/cgo/doc.go

@@ -148,8 +148,9 @@ assignment context to retrieve both the return value (if any) and the
 C errno variable as an error (use _ to skip the result value if the
 function returns void).  For example:
 
-	n, err := C.sqrt(-1)
+	n, err = C.sqrt(-1)
 	_, err := C.voidFunc()
+	var n, err = C.sqrt(1)
 
 Calling C function pointers is currently not supported, however you can
 declare Go variables which hold C function pointers and pass them