Should int(*)() be convertible to inplace_function<void()>?

Before this patch, we reported that `is_convertible<int(*)(), inplace_function<void()>>`,
but if you actually tried to do it, you got a compiler error down in the guts
of `inplace_function`.

After this patch, we let you do it.

However, see issue WG21-SG14#150: I think anyone who relies on this behavior is probably
doing so unintentionally (and their code is probably wrong). Therefore, maybe we
should change our SFINAE so that `is_convertible<int(*)(), inplace_function<void()>>`
would be `false`.

SG14/inplace_function.h

@@ -105,9 +105,11 @@ template<typename R, typename... Args> struct vtable
 
     template<typename C> explicit constexpr vtable(wrapper<C>) noexcept :
         invoke_ptr{ [](storage_ptr_t storage_ptr, Args&&... args) -> R
-            { return (*static_cast<C*>(storage_ptr))(
-                static_cast<Args&&>(args)...
-            ); }
+            {
+                return static_cast<R>((*static_cast<C*>(storage_ptr))(
+                    static_cast<Args&&>(args)...
+                ));
+            }
         },
         copy_ptr{ [](storage_ptr_t dst_ptr, storage_ptr_t src_ptr) -> void
             { ::new (dst_ptr) C{ (*static_cast<C*>(src_ptr)) }; }

SG14_test/inplace_function_test.cpp

@@ -510,6 +510,21 @@ static void test_convertibility_with_lambdas()
     static_assert(!std::is_convertible<decltype(f), stdext::inplace_function<void(int&)>>::value, "");
 }
 
+static void test_void_returning_function_runtime()
+{
+    auto lambda = []() { return 42; };
+    static_assert(std::is_convertible<decltype(lambda), stdext::inplace_function<void()>>::value, "");
+    static_assert(std::is_convertible<decltype(lambda), stdext::inplace_function<const void()>>::value, "");
+
+    stdext::inplace_function<void()> f = lambda;
+    f = lambda;
+    f();
+
+    stdext::inplace_function<const void()> g = lambda;
+    g = lambda;
+    g();
+}
+
 namespace {
 struct InstrumentedCopyConstructor {
     static int copies;
@@ -703,6 +718,7 @@ void sg14_test::inplace_function_test()
     test_is_convertible();
     test_convertibility_with_qualified_call_operators();
     test_convertibility_with_lambdas();
+    test_void_returning_function_runtime();
     test_return_by_move();
     test_is_invocable();
     test_overloading_on_arity();