Better use of std::enable_if (#1177)

* this commits adds tests that should fail, but don't

* Better use of std::enable_if

Replace the occurances of `class = std::enable_if_t<Cond>` and `typename = std::enable_if_t<Cond>` that have been identified in the previous commit with `std::enable_if_t<Cond, bool> = true`.

This commit is inspired by #1174, which changed one occurance in the owner header. This commit is aimed to fix all remaining occurances.

* fix failing checks

- core.cxx_gsl aktualisiert auf [](https://gitlab.avm.de/fos/repos/core.cxx_gsl/-/commit/)
- plc.access_lib aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.access_lib/-/commit/)
- plc.common aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.common/-/commit/)
- plc.daemon aktualisiert auf [](https://gitlab.avm.de/fos/repos/plc.daemon/-/commit/)
-

Test Plan:
-

---------

Co-authored-by: Werner Henze <werner.henze+gitcommits@posteo.de>
Co-authored-by: Werner Henze <w.henze@avm.de>

tests/span_compatibility_tests.cpp

@@ -1005,12 +1005,18 @@ static_assert(std::is_convertible<const std::array<int, 3>&, gsl::span<const int
               "std::is_convertible<const std::array<int, 3>&, gsl::span<const int>>");
 
 #if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 template <typename U, typename = void>
 static constexpr bool AsWritableBytesCompilesFor = false;
 
 template <typename U>
 static constexpr bool
-    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
+    AsWritableBytesCompilesFor<U, ::void_t<decltype(as_writable_bytes(declval<U>()))>> = true;
 
 static_assert(AsWritableBytesCompilesFor<gsl::span<int>>,
               "AsWritableBytesCompilesFor<gsl::span<int>>");
@@ -1020,4 +1026,3 @@ static_assert(!AsWritableBytesCompilesFor<gsl::span<const int>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int>>");
 static_assert(!AsWritableBytesCompilesFor<gsl::span<const int, 9>>,
               "!AsWritableBytesCompilesFor<gsl::span<const int, 9>>");
-#endif // __cplusplus >= 201703l