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/pointers_tests.cpp

@@ -3,6 +3,24 @@
 #include <gsl/pointers>
 
 #include <memory>
+#include <type_traits>
+#include <utility>
+
+namespace
+{
+// Custom pointer type that can be used for gsl::not_null, but for which these cannot be swapped.
+struct NotMoveAssignableCustomPtr
+{
+    NotMoveAssignableCustomPtr() = default;
+    NotMoveAssignableCustomPtr(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr& operator=(const NotMoveAssignableCustomPtr&) = default;
+    NotMoveAssignableCustomPtr(NotMoveAssignableCustomPtr&&) = default;
+    NotMoveAssignableCustomPtr& operator=(NotMoveAssignableCustomPtr&&) = delete;
+
+    bool operator!=(std::nullptr_t) const { return true; }
+
+    int dummy{}; // Without this clang warns, that NotMoveAssignableCustomPtr() is unneeded
+};
 
 TEST(pointers_test, swap)
 {
@@ -17,5 +35,28 @@ TEST(pointers_test, swap)
 
     EXPECT_TRUE(*a == 1);
     EXPECT_TRUE(*b == 0);
+
+    // Make sure our custom ptr can be used with not_null. The shared_pr is to prevent "unused"
+    // compiler warnings.
+    const auto shared_custom_ptr{std::make_shared<NotMoveAssignableCustomPtr>()};
+    gsl::not_null<NotMoveAssignableCustomPtr> c{*shared_custom_ptr};
+    EXPECT_TRUE(c.get() != nullptr);
 }
 
+#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 SwapCompilesFor = false;
+template <typename U>
+static constexpr bool
+    SwapCompilesFor<U, void_t<decltype(gsl::swap<U>(std::declval<gsl::not_null<U>&>(),
+                                                    std::declval<gsl::not_null<U>&>()))>> = true;
+static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
+              "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
+
+} // namespace