[libcxx][test] Niebloids are not portably of `final` class type

[CaseyCarter]

Summary

... nor any of the other things tested by is_function_like, to be fair, but they pass with MSVC STL so :shrug:.

[• Quuxplusone]

I believe this is obsolete since D116384: all uses of is_niebloid are now guarded under LIBCPP_STATIC_ASSERT at their call sites. E.g.

LIBCPP_STATIC_ASSERT(is_niebloid<decltype(std::ranges::prev)>());

That was the superior solution, because actually it's IFNDR even to try to compute decltype(std::ranges::prev) — overload sets don't have decltypes.
Recommend abandoning this.

See also D116570.

[CaseyCarter]

In D117367#3246037, @Quuxplusone wrote:

I believe this is obsolete since D116384: all uses of is_niebloid are now guarded under LIBCPP_STATIC_ASSERT at their call sites. E.g.

:facepalm:, I didn't notice D116384. Abandoning.

libcxx/test/support/is_niebloid.h

constexpr bool is_niebloid() {
 
   static_assert(!std::move_constructible<X>);
   static_assert(!std::assignable_from<X&, X>);
 
   static_assert(!std::copy_constructible<X>);
   static_assert(!std::assignable_from<X&, X const>);
   static_assert(!std::assignable_from<X&, X&>);
   static_assert(!std::assignable_from<X&, X const&>);
-  static_assert(std::is_final_v<X>);
+  LIBCPP_STATIC_ASSERT(std::is_final_v<X>);
   return true;
 }
 #endif
 
 #endif // LIBCXX_TEST_SUPPORT_IS_NIEBLOID_H