Don't test equal_to<void> with different enumerations

Clang 18 SFINAES away the return type in C++23. I've made this compiler-agnostic with the expectation that other compilers will follow suit.

tests/std/tests/GH_000431_equal_family/test.cpp

@@ -244,7 +244,11 @@ void test_algorithms(EqualFn equal_fn) {
         assert(equal_fn(begin(arr2), end(arr2), begin(arr3), end(arr3), equal_to<short>{}));
         assert(equal_fn(begin(arr2), end(arr2), begin(arr3), end(arr3), equal_to<int>{}));
 
+#if !_HAS_CXX23
+        // The return type of equal_to<void>::operator(T, U) SFINAES
+        // when T and U are different enumeration types.
         assert(equal_fn(begin(arr3), end(arr3), begin(arr4), end(arr4), equal_to<>{}));
+#endif // !_HAS_CXX23
         assert(equal_fn(begin(arr3), end(arr3), begin(arr4), end(arr4), equal_to<short>{}));
         assert(equal_fn(begin(arr3), end(arr3), begin(arr4), end(arr4), equal_to<int>{}));
     }