[libcxx] Implement std::to_address for C++20

[glenjofe]

Summary

Now implements P0653R2 - Utility to convert to raw pointer.

[glenjofe]

Include full context in the diff.

[mclow.lists]

This feature has not been added to the draft standard yet, so I'm reluctant to add it to libc++.

I appreciate the patch, and we may adopt it in the future, but not now.

[glenjofe]

Now implements P0653R2 which was accepted in Albuquerque.

[EricWF]

Initially this LGTM. I'll take another pass in the morning.

include/memory

1096

Just make this unconditionally constexpr.

test/std/utilities/memory/pointer.conversion/to_address.pass.cpp

101

Could you add some tests that run at compile time using static_assert.

Also, could you verify the functions are correctly marked noexcept using ASSERT_NOEXCEPT from test_macros.h?

[glenjofe]

Address Eric's review comments.

[EricWF]

test/std/utilities/memory/pointer.conversion/to_address.pass.cpp

119

Shouldn't one of these tests hit a non-noexcept function?

[glenjofe]

test/std/utilities/memory/pointer.conversion/to_address.pass.cpp

119

Both overloads of to_address are unconditionally noexcept.

[glenjofe]

Thanks Eric. Can you commit on my behalf?

include/memory

     static pointer pointer_to(<details>);
 };
 
 template <class T>
 struct pointer_traits<T*>
 {
     typedef T* pointer;
     typedef T element_type;
     typedef ptrdiff_t difference_type;
 
     template <class U> using rebind = U*;
 
     static pointer pointer_to(<details>) noexcept;
 };
 
+template <class T> constexpr T* to_address(T* p) noexcept; // C++20
+template <class Ptr> auto to_address(const Ptr& p) noexcept; // C++20
+
 template <class Alloc>
 struct allocator_traits
 {
     typedef Alloc                        allocator_type;
     typedef typename allocator_type::value_type
                                          value_type;
 
     typedef Alloc::pointer | value_type* pointer;

 #ifndef _LIBCPP_CXX03_LANG
     typedef typename pointer_traits<_Ptr>::template rebind<const void> type;
 #else
     typedef typename pointer_traits<_Ptr>::template rebind<const void>::other type;
 #endif
 };
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp*
 __to_raw_pointer(_Tp* __p) _NOEXCEPT
 {
     return __p;
 }
 
+#if _LIBCPP_STD_VER <= 17
 template <class _Pointer>
 inline _LIBCPP_INLINE_VISIBILITY
 typename pointer_traits<_Pointer>::element_type*
 __to_raw_pointer(_Pointer __p) _NOEXCEPT
 {
     return _VSTD::__to_raw_pointer(__p.operator->());
 }
+#else
+template <class _Pointer>
+inline _LIBCPP_INLINE_VISIBILITY
+auto
+__to_raw_pointer(const _Pointer& __p) _NOEXCEPT
+-> decltype(pointer_traits<_Pointer>::to_address(__p))
+{
+    return pointer_traits<_Pointer>::to_address(__p);
+}
+
+template <class _Pointer, class... _None>
+inline _LIBCPP_INLINE_VISIBILITY
+auto
+__to_raw_pointer(const _Pointer& __p, _None...) _NOEXCEPT
+{
+    return _VSTD::__to_raw_pointer(__p.operator->());
+}
+
+template <class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+_Tp*
+to_address(_Tp* __p) _NOEXCEPT
+{
+    static_assert(!is_function_v<_Tp>, "_Tp is a function type");
+    return __p;
+}
+
+template <class _Pointer>
+inline _LIBCPP_INLINE_VISIBILITY
+auto
+to_address(const _Pointer& __p) _NOEXCEPT
+{
+    return _VSTD::__to_raw_pointer(__p);
+}
+#endif
 
 template <class _Tp, class = void>
 struct __has_size_type : false_type {};
 
 template <class _Tp>
 struct __has_size_type<_Tp,
                typename __void_t<typename _Tp::size_type>::type> : true_type {};
 
 template <class _Alloc, class _DiffType, bool = __has_size_type<_Alloc>::value>
 struct __size_type
 {
     typedef typename make_unsigned<_DiffType>::type type;
 };
 
 template <class _Alloc, class _DiffType>

test/std/utilities/memory/pointer.conversion/to_address.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// template <class T> constexpr T* to_address(T* p) noexcept;
+// template <class Ptr> auto to_address(const Ptr& p) noexcept;
+
+#include <memory>
+#include <cassert>
+#include "test_macros.h"
+
+class P1
+{
+public:
+    using element_type = int;
+
+    explicit P1(int* p)
+    : p_(p) { }
+
+    int* operator->() const noexcept
+    { return p_; }
+
+private:
+    int* p_;
+};
+
+class P2
+{
+public:
+    using element_type = int;
+
+    explicit P2(int* p)
+    : p_(p) { }
+
+    P1 operator->() const noexcept
+    { return p_; }
+
+private:
+    P1 p_;
+};
+
+class P3
+{
+public:
+    explicit P3(int* p)
+    : p_(p) { }
+
+    int* get() const noexcept
+    { return p_; }
+
+private:
+    int* p_;
+};
+
+namespace std
+{
+template<>
+struct pointer_traits<::P3>
+{
+    static int* to_address(const ::P3& p) noexcept
+    { return p.get(); }
+};
+}
+
+class P4
+{
+public:
+    explicit P4(int* p)
+    : p_(p) { }
+
+    int* operator->() const noexcept
+    { return nullptr; }
+
+    int* get() const noexcept
+    { return p_; }
+
+private:
+    int* p_;
+};
+
+namespace std
+{
+template<>
+struct pointer_traits<::P4>
+{
+    static int* to_address(const ::P4& p) noexcept
+    { return p.get(); }
+};
+}
+
+int n = 0;
+static_assert(std::to_address(&n) == &n);
+
+int main()
+{
+    int i = 0;
+    ASSERT_NOEXCEPT(std::to_address(&i));
+    assert(std::to_address(&i) == &i);
+    P1 p1(&i);
+    ASSERT_NOEXCEPT(std::to_address(p1));
+    assert(std::to_address(p1) == &i);
+    P2 p2(&i);
+    ASSERT_NOEXCEPT(std::to_address(p2));
+    assert(std::to_address(p2) == &i);
+    P3 p3(&i);
+    ASSERT_NOEXCEPT(std::to_address(p3));
+    assert(std::to_address(p3) == &i);
+    P4 p4(&i);
+    ASSERT_NOEXCEPT(std::to_address(p4));
+    assert(std::to_address(p4) == &i);
+}