Fix is_invocable_v and add test coverage

Author: badair

include/boost/callable_traits/is_invocable.hpp

@@ -67,7 +67,7 @@ BOOST_CLBL_TRAITS_INLINE_VAR
 //->
 constexpr bool is_invocable_v = //see below
 //<-
-    detail::is_invocable_impl<detail::traits<T>, Args...>::type::value;
+    detail::is_invocable_impl<T, Args...>::type::value;
 //->
 
 // only available when variable templates are supported

test/is_invocable.cpp

@@ -28,8 +28,14 @@ struct invoke_case {
 // when available, test parity with std implementation (c++2a breaks our expectations but we still match std impl)
 #if defined(__cpp_lib_is_invocable) || __cplusplus >= 201707L
        CT_ASSERT((std::is_invocable<Callable, Args...>() == boost::callable_traits::is_invocable<Callable, Args...>()));
+#  ifndef BOOST_CLBL_TRTS_DISABLE_VARIABLE_TEMPLATES
+       CT_ASSERT((std::is_invocable_v<Callable, Args...> == boost::callable_traits::is_invocable_v<Callable, Args...>));
+#  endif
 #else
        CT_ASSERT((Expect == boost::callable_traits::is_invocable<Callable, Args...>()));
+#  ifndef BOOST_CLBL_TRTS_DISABLE_VARIABLE_TEMPLATES
+       CT_ASSERT((Expect == boost::callable_traits::is_invocable_v<Callable, Args...>));
+#  endif
 #endif
 
    }

test/is_invocable_r.cpp

@@ -28,8 +28,14 @@ struct invoke_case {
 // when available, test parity with std implementation (c++2a breaks our expectations but we still match std impl)
 #if defined(__cpp_lib_is_invocable) || __cplusplus >= 201707L
         CT_ASSERT((std::is_invocable_r<Ret, Callable, Args...>() == boost::callable_traits::is_invocable_r<Ret, Callable, Args...>()));
+#  ifndef BOOST_CLBL_TRTS_DISABLE_VARIABLE_TEMPLATES
+        CT_ASSERT((std::is_invocable_r_v<Ret, Callable, Args...> == boost::callable_traits::is_invocable_r_v<Ret, Callable, Args...>));
+#  endif
 #else
         CT_ASSERT((Expect == boost::callable_traits::is_invocable_r<Ret, Callable, Args...>()));
+#  ifndef BOOST_CLBL_TRTS_DISABLE_VARIABLE_TEMPLATES
+        CT_ASSERT((Expect == boost::callable_traits::is_invocable_r_v<Ret, Callable, Args...>));
+#  endif
 #endif
 
    }