More checks for non-compilable code, plus fix for span (#1180)

tests/pointers_tests.cpp

@@ -6,6 +6,13 @@
 #include <type_traits>
 #include <utility>
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 namespace
 {
 // Custom pointer type that can be used for gsl::not_null, but for which these cannot be swapped.
@@ -22,6 +29,13 @@ struct NotMoveAssignableCustomPtr
     int dummy{}; // Without this clang warns, that NotMoveAssignableCustomPtr() is unneeded
 };
 
+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;
+
 TEST(pointers_test, swap)
 {
     // taken from gh-1129:
@@ -69,22 +83,9 @@ TEST(pointers_test, swap)
         EXPECT_TRUE(*a == 1);
         EXPECT_TRUE(*b == 0);
     }
+
+    static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
+                  "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
 }
 
-#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