[libc++] Implement LWG3545: std::pointer_traits should be SFINAE-friendly.

[zetafunction]

Summary

See https://wg21.link/LWG3545 for background and details.

[zetafunction]

Make sure pointer_traits.h is C++03-compatible.

[zetafunction]

Use {} for aggregate init for compat with older compilers.

[philnik]

Since we are moving to GitHub PRs and there hasn't been any activity here yet, could you move this patch over?

[zetafunction]

Is there updated documentation for best practices for doing that? I was following https://libcxx.llvm.org/Contributing.html.

[philnik]

In D158922#4634975, @zetafunction wrote:

Is there updated documentation for best practices for doing that? I was following https://libcxx.llvm.org/Contributing.html.

We have just started the move, so I don't think there is any documentation yet. You should be able to just fork the llvm-project and then open a PR against the main repo.

[zetafunction]

In D158922#4635027, @philnik wrote:

In D158922#4634975, @zetafunction wrote:

Is there updated documentation for best practices for doing that? I was following https://libcxx.llvm.org/Contributing.html.

We have just started the move, so I don't think there is any documentation yet. You should be able to just fork the llvm-project and then open a PR against the main repo.

Thanks, I've gone ahead and opened the PR at https://github.com/llvm/llvm-project/pull/65177? Since you commented, would you be willing to take a look at it? It hasn't been quite a week yet (per https://llvm.org/docs/Contributing.html#how-to-submit-a-patch) but I'm not sure what the best way to proceed here is either, since this is my first libc++ patch.

[philnik]

In D158922#4635043, @zetafunction wrote:

In D158922#4635027, @philnik wrote:

In D158922#4634975, @zetafunction wrote:

Is there updated documentation for best practices for doing that? I was following https://libcxx.llvm.org/Contributing.html.

We have just started the move, so I don't think there is any documentation yet. You should be able to just fork the llvm-project and then open a PR against the main repo.

Thanks, I've gone ahead and opened the PR at https://github.com/llvm/llvm-project/pull/65177? Since you commented, would you be willing to take a look at it? It hasn't been quite a week yet (per https://llvm.org/docs/Contributing.html#how-to-submit-a-patch) but I'm not sure what the best way to proceed here is either, since this is my first libc++ patch.

Reviews are quite slow currently because we are still working on the 17 release and the GitHub PR merge is ongoing and utilizing almost all the resources we have. I'm currently also not very active, making matters worse. The review speed will probably pick up again in a week or two.

Please abandon this patch, since it's on GitHub now.

libcxx/docs/Status/Cxx23Issues.csv

 "`3719 <https://wg21.link/LWG3719>`__","Directory iterators should be usable with default sentinel","July 2022","|Complete|","17.0","|ranges|"
 "`3721 <https://wg21.link/LWG3721>`__","Allow an ``arg-id`` with a value of zero for ``width`` in ``std-format-spec``","July 2022","|Complete|","16.0","|format|"
 "`3724 <https://wg21.link/LWG3724>`__","``decay-copy`` should be constrained","July 2022","|Complete|","14.0"
 "","","","","",""
 "`3028 <https://wg21.link/LWG3028>`__","Container requirements tables should distinguish ``const`` and non-``const`` variables", "November 2022","","",""
 "`3118 <https://wg21.link/LWG3118>`__","``fpos`` equality comparison unspecified", "November 2022","","",""
 "`3177 <https://wg21.link/LWG3177>`__","Limit permission to specialize variable templates to program-defined types", "November 2022","|Nothing to do|","",""
 "`3515 <https://wg21.link/LWG3515>`__","§[stacktrace.basic.nonmem]: ``operator<<`` should be less templatized", "November 2022","","",""
-"`3545 <https://wg21.link/LWG3545>`__","``std::pointer_traits`` should be SFINAE-friendly", "November 2022","","",""
+"`3545 <https://wg21.link/LWG3545>`__","``std::pointer_traits`` should be SFINAE-friendly", "November 2022","|Complete|","18.0",""
 "`3569 <https://wg21.link/LWG3569>`__","``join_view`` fails to support ranges of ranges with non-default_initializable iterators", "November 2022","","","|ranges|"
 "`3594 <https://wg21.link/LWG3594>`__","``inout_ptr`` — inconsistent ``release()`` in destructor", "November 2022","","",""
 "`3597 <https://wg21.link/LWG3597>`__","Unsigned integer types don't model advanceable", "November 2022","","","|ranges|"
 "`3600 <https://wg21.link/LWG3600>`__","Making ``istream_iterator`` copy constructor trivial is an ABI break", "November 2022","","",""
 "`3629 <https://wg21.link/LWG3629>`__","``make_error_code`` and ``make_error_condition`` are customization points","November 2022","|Complete|","16.0",""
 "`3636 <https://wg21.link/LWG3636>`__","``formatter<T>::format`` should be ``const``-qualified","November 2022","|Complete|","16.0","|format|"
 "`3646 <https://wg21.link/LWG3646>`__","``std::ranges::view_interface::size`` returns a signed type","November 2022","|Complete|","16.0","|ranges|"
 "`3677 <https://wg21.link/LWG3677>`__","Is a cv-qualified ``pair`` specially handled in uses-allocator construction?", "November 2022","","",""

libcxx/include/__memory/pointer_traits.h

 
 template <class _Tp, class = void>
 struct __has_element_type : false_type {};
 
 template <class _Tp>
 struct __has_element_type<_Tp, __void_t<typename _Tp::element_type> > : true_type {};
 
 template <class _Ptr, bool = __has_element_type<_Ptr>::value>
-struct __pointer_traits_element_type;
+struct __pointer_traits_element_type {};
 
 template <class _Ptr>
 struct __pointer_traits_element_type<_Ptr, true>
 {
     typedef _LIBCPP_NODEBUG typename _Ptr::element_type type;
 };
 
 template <template <class, class...> class _Sp, class _Tp, class ..._Args>

 };
 
 template <template <class, class...> class _Sp, class _Tp, class ..._Args, class _Up>
 struct __pointer_traits_rebind<_Sp<_Tp, _Args...>, _Up, false>
 {
     typedef _Sp<_Up, _Args...> type;
 };
 
+template <class _Ptr, class = void>
+struct __pointer_traits_impl {};
+
 template <class _Ptr>
-struct _LIBCPP_TEMPLATE_VIS pointer_traits
-{
-    typedef _Ptr                                                     pointer;
-    typedef typename __pointer_traits_element_type<pointer>::type    element_type;
-    typedef typename __pointer_traits_difference_type<pointer>::type difference_type;
+struct __pointer_traits_impl<_Ptr, __void_t<typename __pointer_traits_element_type<_Ptr>::type> > {
+  typedef _Ptr pointer;
+  typedef typename __pointer_traits_element_type<pointer>::type element_type;
 
 #ifndef _LIBCPP_CXX03_LANG
     template <class _Up> using rebind = typename __pointer_traits_rebind<pointer, _Up>::type;
 #else
     template <class _Up> struct rebind
         {typedef typename __pointer_traits_rebind<pointer, _Up>::type other;};
 #endif // _LIBCPP_CXX03_LANG
 
 private:
     struct __nat {};
 public:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_SINCE_CXX20
     static pointer pointer_to(__conditional_t<is_void<element_type>::value, __nat, element_type>& __r)
         {return pointer::pointer_to(__r);}
 };
 
+template <class _Ptr>
+struct _LIBCPP_TEMPLATE_VIS pointer_traits : __pointer_traits_impl<_Ptr> {};
+
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS pointer_traits<_Tp*>
 {
     typedef _Tp*      pointer;
     typedef _Tp       element_type;
     typedef ptrdiff_t difference_type;
 
 #ifndef _LIBCPP_CXX03_LANG

libcxx/test/libcxx/iterators/iterator.requirements/iterator.concepts/iterator.concept.random.access/contiguous_iterator.verify.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-
-// This test checks that std::contiguous_iterator uses std::to_address, which is not SFINAE-friendly
-// when the type is missing the `T::element_type` typedef.
-
-#include <iterator>
-
-#include <compare>
-#include <cstddef>
-
-struct no_element_type {
-    typedef std::contiguous_iterator_tag    iterator_category;
-    typedef int                             value_type;
-    typedef std::ptrdiff_t                  difference_type;
-    typedef int*                            pointer;
-    typedef int&                            reference;
-    typedef no_element_type                 self;
-
-    no_element_type();
-
-    reference operator*() const;
-    pointer operator->() const;
-    auto operator<=>(const self&) const = default;
-
-    self& operator++();
-    self operator++(int);
-
-    self& operator--();
-    self operator--(int);
-
-    self& operator+=(difference_type n);
-    self operator+(difference_type n) const;
-    friend self operator+(difference_type n, self x);
-
-    self& operator-=(difference_type n);
-    self operator-(difference_type n) const;
-    difference_type operator-(const self& n) const;
-
-    reference operator[](difference_type n) const;
-};
-
-void test() {
-    (void) std::contiguous_iterator<no_element_type>;
-        // expected-error@*:* {{implicit instantiation of undefined template}}
-        // expected-note@*:* {{to_address}}
-}

libcxx/test/std/utilities/memory/pointer.conversion/to_address.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// template <class Ptr> constexpr auto to_address(const Ptr& p) noexcept;
+//     Mandates: one of pointer_traits<Ptr>::to_address() or Ptr::operator->()
+//     is present.
+
+#include <memory>
+
+struct NotPtr {};
+
+void test() {
+  (void)std::to_address(NotPtr()); // expected-error@*:* {{no matching function for call to 'to_address'}}
+}

libcxx/test/std/utilities/memory/pointer.conversion/to_address_without_pointer_traits.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <memory>
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// template <class Ptr> constexpr auto to_address(const Ptr& p) noexcept;
+//     Should not require a specialization of pointer_traits for Ptr.
+
+#include <memory>
+
+struct IntPtr {
+  constexpr int* operator->() const { return ptr; }
+
+  int* ptr;
+};
+
+template <typename T, bool>
+struct TemplatedPtr {
+  constexpr T* operator->() const { return ptr; }
+
+  T* ptr;
+};
+
+constexpr bool test() {
+  int i = 0;
+
+  static_assert(std::to_address(IntPtr{nullptr}) == nullptr);
+  static_assert(std::to_address(IntPtr{&i}) == &i);
+
+  bool b = false;
+
+  static_assert(std::to_address(TemplatedPtr<bool, true>{nullptr}) == nullptr);
+  static_assert(std::to_address(TemplatedPtr<bool, true>{&b}) == &b);
+
+  return true;
+}
+
+int main(int, char**) {
+  static_assert(test());
+  return 0;
+}