Fix eagerness bug in FOLLY_CREATE_STATIC_MEMBER_INVOKER

Summary:
This patch performs SFINAE-friendly invocation in `FOLLY_CREATE_STATIC_MEMBER_INVOKER` in order for invocability tests to return `false` in the situation where the member is not even available. For example, given:
```
FOLLY_CREATE_STATIC_MEMBER_INVOKER(foo_invoker, foo);
```
With this diff, something like `std::is_invocable_v<foo_invoker, HasNoFoo>` yields `false` rather than a compilation error.
This brings `FOLLY_CREATE_STATIC_MEMBER_INVOKER` in line with `FOLLY_CREATE_FREE_INVOKER` and `FOLLY_CREATE_MEMBER_INVOKER`.

Reviewed By: yfeldblum

Differential Revision: D25878262

fbshipit-source-id: 150ae6b2c71941317bc02de20720fed014639bf5

folly/functional/Invoke.h

@@ -476,11 +476,11 @@ struct invoke_traits : detail::invoke_traits_base<I> {
 #define FOLLY_CREATE_STATIC_MEMBER_INVOKER(classname, membername)             \
   template <typename T>                                                       \
   struct classname {                                                          \
-    template <typename... Args>                                               \
+    template <typename... Args, typename U = T>                               \
     FOLLY_MAYBE_UNUSED FOLLY_ERASE constexpr auto operator()(Args&&... args)  \
-        const noexcept(noexcept(T::membername(static_cast<Args&&>(args)...))) \
-            -> decltype(T::membername(static_cast<Args&&>(args)...)) {        \
-      return T::membername(static_cast<Args&&>(args)...);                     \
+        const noexcept(noexcept(U::membername(static_cast<Args&&>(args)...))) \
+            -> decltype(U::membername(static_cast<Args&&>(args)...)) {        \
+      return U::membername(static_cast<Args&&>(args)...);                     \
     }                                                                         \
   }
 

folly/functional/test/InvokeTest.cpp

@@ -298,6 +298,24 @@ TEST_F(InvokeTest, static_member_invoke) {
   EXPECT_FALSE((traits::is_nothrow_invocable_r_v<int, int>));
 }
 
+TEST_F(InvokeTest, static_member_no_invoke) {
+  struct HasNoStat {};
+
+  using traits = folly::invoke_traits<stat_invoker<HasNoStat>>;
+
+  EXPECT_FALSE((traits::is_invocable_v<>));
+  EXPECT_FALSE((traits::is_invocable_v<int>));
+
+  EXPECT_FALSE((traits::is_invocable_r_v<int>));
+  EXPECT_FALSE((traits::is_invocable_r_v<int, int>));
+
+  EXPECT_FALSE((traits::is_nothrow_invocable_v<>));
+  EXPECT_FALSE((traits::is_nothrow_invocable_v<int>));
+
+  EXPECT_FALSE((traits::is_nothrow_invocable_r_v<int>));
+  EXPECT_FALSE((traits::is_nothrow_invocable_r_v<int, int>));
+}
+
 namespace {
 
 struct TestCustomisationPointFn {