[libcxx-commits] [libcxx] [libc++] P3450R1: Extend `std::is_within_lifetime` (PR #201053)

[A. Jiang via libcxx-commits]

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@@ -18,9 +18,9 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 26 && __has_builtin(__builtin_is_within_lifetime)
-template <class _Tp>
+template <class _Up = void, class _Tp>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI consteval bool is_within_lifetime(const _Tp* __p) noexcept {
-  return __builtin_is_within_lifetime(__p);
+  return __builtin_is_within_lifetime(__p) && __builtin_constant_p(static_cast<const volatile _Up*>(__p) && true);
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frederick-vs-ja wrote:

> Wouldn't it be better to use this implementation as a temporary workaround and to update the builtin itself to provide the updated functionality?

I think we can treat use of `__builtin_constant_p` as a workaround, and currently this PR is doing so.

> Are there any other uses for `__builtin_is_within_lifetime` that require just the initial functionality?

No, I think. `__builtin_is_within_lifetime` is quite specific to `std::is_within_lifetime`.

> I'd find it strange that the builtin with the same name as std::is_within_lifetime supports just some of it.

Agree. I just wanted to enable the library change without compiler change. Also, it seems to me that we can change less code in the whole LLVM codebase with the workaround.

https://github.com/llvm/llvm-project/pull/201053