[clang] [Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (PR #91895)

[Louis Dionne via cfe-commits]

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@@ -0,0 +1,431 @@
+// RUN: %clang_cc1 -std=c++20 -Wno-unused %s -verify=expected,cxx20 -Wno-vla-cxx-extension
+// RUN: %clang_cc1 -std=c++23 -Wno-unused %s -verify=expected,sincecxx23 -Wno-vla-cxx-extension
+// RUN: %clang_cc1 -std=c++26 -Wno-unused %s -verify=expected,sincecxx23 -Wno-vla-cxx-extension
+// RUN: %clang_cc1 -std=c++26 -DINLINE_NAMESPACE -Wno-unused %s -verify=expected,sincecxx23 -Wno-vla-cxx-extension
+
+inline constexpr void* operator new(__SIZE_TYPE__, void* p) noexcept { return p; }
+namespace std {
+template<typename T, typename... Args>
+constexpr T* construct_at(T* p, Args&&... args) { return ::new((void*)p) T(static_cast<Args&&>(args)...); }
+template<typename T>
+constexpr void destroy_at(T* p) { p->~T(); }
+template<typename T>
+struct allocator {
+  constexpr T* allocate(__SIZE_TYPE__ n) { return static_cast<T*>(::operator new(n * sizeof(T))); }
+  constexpr void deallocate(T* p, __SIZE_TYPE__) { ::operator delete(p); }
+};
+using nullptr_t = decltype(nullptr);
+template<typename T, T v>
+struct integral_constant { static constexpr T value = v; };
+template<bool v>
+using bool_constant = integral_constant<bool, v>;
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+template<typename T>
+inline constexpr bool is_function_v = __is_function(T);
+#ifdef INLINE_NAMESPACE
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ldionne wrote:

I didn't read super carefully, but can you explain what's the purpose of the inline namespace here?

https://github.com/llvm/llvm-project/pull/91895