Make Function shared proxy empty when empty

Summary: [Folly] Make `Function` shared proxy empty when constructed with an empty `Function`, just like it would be when constructed with `nullptr`.

Reviewed By: ericniebler, vitaut

Differential Revision: D18518858

fbshipit-source-id: fdad5b4cd30a1fda5465dae80054c5c74837faa9

folly/Function.h

@@ -328,7 +328,8 @@ class FunctionTraitsSharedProxy {
  public:
   explicit FunctionTraitsSharedProxy(std::nullptr_t) noexcept {}
   explicit FunctionTraitsSharedProxy(Function<F>&& func)
-      : sp_(std::make_shared<Function<F>>(std::move(func))) {}
+      : sp_(func ? std::make_shared<Function<F>>(std::move(func))
+                 : std::shared_ptr<Function<F>>()) {}
   R operator()(A&&... args) const {
     if (!sp_) {
       throw_exception<std::bad_function_call>();

folly/test/FunctionTest.cpp

@@ -1097,6 +1097,12 @@ TEST(Function, asSharedProxy_nullptr) {
   EXPECT_THROW(sp(3, 4), std::bad_function_call);
 }
 
+TEST(Function, asSharedProxy_empty) {
+  auto func = folly::Function<int(int, int) const>();
+  auto sp = std::move(func).asSharedProxy();
+  EXPECT_THROW(sp(3, 4), std::bad_function_call);
+}
+
 TEST(Function, NoAllocatedMemoryAfterMove) {
   Functor<int, 100> foo;