[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:

`__builtin_constant_p` will always gives `false` if we use `__builtin_constant_p(static_cast<const volatile _Up*>(__p))` as-is. I think this is because of that `__builtin_constant_p` checks whether the final result is a constant expression, not merely where the operand is constant-evaluable.

`&& false`, `|| true`, `|| false`, `? 0 : 0`, and `((void)(expr), 0)` equivalently work.

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