Solve WG21-SG14#149, except for the part in WG21-SG14#150 (which is a…

…n important part).

SG14/inplace_function.h

@@ -150,6 +150,41 @@ struct is_valid_inplace_dst : std::true_type
     );
 };
 
+#if __cpp_lib_is_invocable >= 201703L
+using std::is_invocable_r;
+#else
+template<class Void, class F, class... Args>
+struct is_invocable_helper : std::false_type {};
+
+template<class F, class... Args>
+struct is_invocable_helper<decltype(std::declval<F()>()()(std::declval<Args>()...), void()), F, Args...> : std::true_type {};
+
+template<class F, class... Args>
+struct is_invocable : is_invocable_helper<void, F, Args...> {};
+
+template<class Void, class R, class F, class... Args>
+struct is_invocable_r_helper : std::false_type {};
+
+template<class R, class F, class... Args>
+struct is_invocable_r_helper<decltype(std::declval<F()>()()(std::declval<Args>()...), void()), R, F, Args...> : std::is_convertible<decltype(std::declval<F()>()()(std::declval<Args>()...)), R> {};
+
+template<class R, class F, class... Args>
+struct is_invocable_r : is_invocable_r_helper<void, R, F, Args...> {};
+
+template<class F, class... Args> struct is_invocable_r<void, F, Args...> : is_invocable<F, Args...> {};
+template<class F, class... Args> struct is_invocable_r<const void, F, Args...> : is_invocable<F, Args...> {};
+template<class F, class... Args> struct is_invocable_r<volatile void, F, Args...> : is_invocable<F, Args...> {};
+template<class F, class... Args> struct is_invocable_r<const volatile void, F, Args...> : is_invocable<F, Args...> {};
+
+static_assert(is_invocable<int(*)(int), int>::value, "sanity check");
+static_assert(is_invocable_r<int*, int*(*)(int), int>::value, "sanity check");
+static_assert(is_invocable_r<void, int*(*)(int), int>::value, "sanity check");
+static_assert(is_invocable_r<void, void(*)(int), int>::value, "sanity check");
+static_assert(!is_invocable_r<int, void(*)(int), int>::value, "sanity check");
+static_assert(!is_invocable_r<void, void(*)(int)>::value, "sanity check");
+static_assert(is_invocable_r<void, void(*)()>::value, "sanity check");
+#endif
+
 } // namespace inplace_function_detail
 
 template<
@@ -187,12 +222,16 @@ class inplace_function<R(Args...), Capacity, Alignment>
         typename = std::enable_if_t<
             !(std::is_same<C, inplace_function>::value
             || std::is_convertible<C, inplace_function>::value)
+        >,
+        typename = std::enable_if_t<
+            inplace_function_detail::is_invocable_r<R, C&, Args...>::value
         >
     >
     inplace_function(T&& closure)
     {
-#if __cplusplus >= 201703L
-        static_assert(std::is_invocable_r<R, C, Args...>::value,
+#if __cpp_lib_is_invocable >= 201703L
+        static_assert(std::is_invocable_r<R, C&, Args...>::value,
+            // SFINAE should prevent this assertion from ever triggering anymore.
             "inplace_function cannot be constructed from non-callable type"
         );
 #endif

SG14_test/inplace_function_test.cpp

@@ -414,6 +414,35 @@ static void test_void_returning_function()
     f();
 }
 
+template<class FirstSig, class OtherSig>
+struct test_sfinae_helper {
+    static bool accepts(stdext::inplace_function<FirstSig>) {
+        return true;
+    }
+    static bool accepts(stdext::inplace_function<OtherSig>) {
+        return false;
+    }
+};
+
+void test_sfinae_on_noncallable()
+{
+    if (true) {
+        test_sfinae_helper<void(), void(size_t)> tester;
+        assert(tester.accepts([](){ }) == true);
+        assert(tester.accepts([](size_t){ }) == false);
+    }
+    if (true) {
+        test_sfinae_helper<int*(), int()> tester;
+        assert(tester.accepts([](){ return nullptr; }) == true);
+        assert(tester.accepts([](){ return 42; }) == false);
+    }
+    if (true) {
+        test_sfinae_helper<void(), int()> tester;
+        assert(tester.accepts([](){ return; }) == true);
+        assert(tester.accepts([](){ return nullptr; }) == true);
+    }
+}
+
 void sg14_test::inplace_function_test()
 {
     // first set of tests (from Optiver)
@@ -497,6 +526,7 @@ void sg14_test::inplace_function_test()
     test_overloaded_operator_new();
     test_void_returning_function();
     test_move_construction_is_noexcept();
+    test_sfinae_on_noncallable();
 }
 
 #ifdef TEST_MAIN