folly | Add explicit bool conversion operator to Function::SharedProxy

Summary: `folly::Function` has this explicit bool conversion operator and it makes sense to also have it for `SharedProxy`.

Reviewed By: yfeldblum, ericniebler

Differential Revision: D18504667

fbshipit-source-id: 32fede326c7a3eebd340dea9d118714a0cdfa5c0

folly/Function.h

@@ -337,6 +337,10 @@ class FunctionTraitsSharedProxy {
     return (*sp_)(static_cast<A&&>(args)...);
   }
 
+  explicit operator bool() const noexcept {
+    return sp_ != nullptr;
+  }
+
   friend bool operator==(
       FunctionTraitsSharedProxy<F, R, A...> const& proxy,
       std::nullptr_t) noexcept {

folly/test/FunctionTest.cpp

@@ -1103,6 +1103,20 @@ TEST(Function, asSharedProxy_empty) {
   EXPECT_THROW(sp(3, 4), std::bad_function_call);
 }
 
+TEST(Function, asSharedProxy_explicit_bool_conversion) {
+  folly::Function<void(void)> f = []() {};
+  auto sp = std::move(f).asSharedProxy();
+  auto spcopy = sp;
+  EXPECT_TRUE(sp);
+  EXPECT_TRUE(spcopy);
+
+  folly::Function<void(void)> emptyF;
+  auto emptySp = std::move(emptyF).asSharedProxy();
+  auto emptySpcopy = emptySp;
+  EXPECT_FALSE(emptySp);
+  EXPECT_FALSE(emptySpcopy);
+}
+
 TEST(Function, NoAllocatedMemoryAfterMove) {
   Functor<int, 100> foo;