[libc++] Refactor __tuple_like and __pair_like (#85206)

The exposition-only type trait `pair-like` includes `ranges::subrange`,
but in every single case excludes `ranges::subrange` from the list. This
patch introduces two new traits `__tuple_like_no_subrange` and
`__pair_like_no_subrange`, which exclude `ranges::subrange` from the
possible matches. `__pair_like` is no longer required, and thus removed.
`__tuple_like` is implemented as `__tuple_like_no_subrange` or a
`ranges::subrange` specialization.

Author: philnik777

libcxx/include/CMakeLists.txt

@@ -709,12 +709,12 @@ set(files
   __tree
   __tuple/find_index.h
   __tuple/make_tuple_types.h
-  __tuple/pair_like.h
   __tuple/sfinae_helpers.h
   __tuple/tuple_element.h
   __tuple/tuple_indices.h
   __tuple/tuple_like.h
   __tuple/tuple_like_ext.h
+  __tuple/tuple_like_no_subrange.h
   __tuple/tuple_size.h
   __tuple/tuple_types.h
   __type_traits/add_const.h

libcxx/include/__algorithm/mismatch.h

@@ -18,10 +18,12 @@
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_equality_comparable.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/operation_traits.h>
 #include <__utility/move.h>
 #include <__utility/pair.h>
 #include <__utility/unreachable.h>
+#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__memory/uses_allocator_construction.h

@@ -12,7 +12,7 @@
 #include <__config>
 #include <__memory/construct_at.h>
 #include <__memory/uses_allocator.h>
-#include <__tuple/pair_like.h>
+#include <__tuple/tuple_like_no_subrange.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/remove_cv.h>
@@ -128,11 +128,7 @@ __uses_allocator_construction_args(const _Alloc& __alloc, const pair<_Up, _Vp>&&
       std::forward_as_tuple(std::get<1>(std::move(__pair))));
 }
 
-template < class _Pair,
-           class _Alloc,
-           __pair_like _PairLike,
-           __enable_if_t<__is_cv_std_pair<_Pair> && !__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value,
-                         int> = 0>
+template <class _Pair, class _Alloc, __pair_like_no_subrange _PairLike, __enable_if_t<__is_cv_std_pair<_Pair>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI constexpr auto
 __uses_allocator_construction_args(const _Alloc& __alloc, _PairLike&& __p) noexcept {
   return std::__uses_allocator_construction_args<_Pair>(
@@ -161,9 +157,7 @@ inline constexpr bool __convertible_to_const_pair_ref =
 #  if _LIBCPP_STD_VER >= 23
 template <class _Tp, class _Up>
 inline constexpr bool __uses_allocator_constraints =
-    __is_cv_std_pair<_Tp> &&
-    (__is_specialization_of_subrange<remove_cvref_t<_Up>>::value ||
-     (!__pair_like<_Up> && !__convertible_to_const_pair_ref<_Up>));
+    __is_cv_std_pair<_Tp> && !__pair_like_no_subrange<_Up> && !__convertible_to_const_pair_ref<_Up>;
 #  else
 template <class _Tp, class _Up>
 inline constexpr bool __uses_allocator_constraints = __is_cv_std_pair<_Tp> && !__convertible_to_const_pair_ref<_Up>;

libcxx/include/__memory_resource/polymorphic_allocator.h

@@ -12,6 +12,7 @@
 #include <__assert>
 #include <__availability>
 #include <__config>
+#include <__fwd/pair.h>
 #include <__memory_resource/memory_resource.h>
 #include <__utility/exception_guard.h>
 #include <cstddef>

libcxx/include/__ranges/subrange.h

@@ -28,8 +28,8 @@
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/size.h>
 #include <__ranges/view_interface.h>
-#include <__tuple/pair_like.h>
 #include <__tuple/tuple_element.h>
+#include <__tuple/tuple_like_no_subrange.h>
 #include <__tuple/tuple_size.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/decay.h>
@@ -64,7 +64,7 @@ concept __convertible_to_non_slicing =
 
 template <class _Pair, class _Iter, class _Sent>
 concept __pair_like_convertible_from =
-    !range<_Pair> && __pair_like<_Pair> && constructible_from<_Pair, _Iter, _Sent> &&
+    !range<_Pair> && __pair_like_no_subrange<_Pair> && constructible_from<_Pair, _Iter, _Sent> &&
     __convertible_to_non_slicing<_Iter, tuple_element_t<0, _Pair>> && convertible_to<_Sent, tuple_element_t<1, _Pair>>;
 
 template <input_or_output_iterator _Iter,
@@ -125,8 +125,7 @@ class _LIBCPP_TEMPLATE_VIS subrange : public view_interface<subrange<_Iter, _Sen
                requires(_Kind == subrange_kind::sized)
       : subrange(ranges::begin(__range), ranges::end(__range), __n) {}
 
-  template <__different_from<subrange> _Pair>
-    requires __pair_like_convertible_from<_Pair, const _Iter&, const _Sent&>
+  template <__pair_like_convertible_from<const _Iter&, const _Sent&> _Pair>
   _LIBCPP_HIDE_FROM_ABI constexpr operator _Pair() const {
     return _Pair(__begin_, __end_);
   }

libcxx/include/__tuple/pair_like.h

@@ -10,12 +10,9 @@
 #define _LIBCPP___TUPLE_TUPLE_LIKE_H
 
 #include <__config>
-#include <__fwd/array.h>
-#include <__fwd/complex.h>
-#include <__fwd/pair.h>
 #include <__fwd/subrange.h>
-#include <__fwd/tuple.h>
-#include <__type_traits/integral_constant.h>
+#include <__tuple/tuple_like_no_subrange.h>
+#include <__tuple/tuple_size.h>
 #include <__type_traits/remove_cvref.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -27,29 +24,16 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp>
-struct __tuple_like_impl : false_type {};
+inline constexpr bool __is_ranges_subrange_v = false;
 
-template <class... _Tp>
-struct __tuple_like_impl<tuple<_Tp...> > : true_type {};
-
-template <class _T1, class _T2>
-struct __tuple_like_impl<pair<_T1, _T2> > : true_type {};
-
-template <class _Tp, size_t _Size>
-struct __tuple_like_impl<array<_Tp, _Size> > : true_type {};
-
-template <class _Ip, class _Sp, ranges::subrange_kind _Kp>
-struct __tuple_like_impl<ranges::subrange<_Ip, _Sp, _Kp> > : true_type {};
-
-#  if _LIBCPP_STD_VER >= 26
+template <class _Iter, class _Sent, ranges::subrange_kind _Kind>
+inline constexpr bool __is_ranges_subrange_v<ranges::subrange<_Iter, _Sent, _Kind>> = true;
 
 template <class _Tp>
-struct __tuple_like_impl<complex<_Tp>> : true_type {};
-
-#  endif
+concept __tuple_like = __tuple_like_no_subrange<_Tp> || __is_ranges_subrange_v<remove_cvref_t<_Tp>>;
 
-template <class _Tp>
-concept __tuple_like = __tuple_like_impl<remove_cvref_t<_Tp>>::value;
+// As of writing this comment every use of `pair-like` in the standard excludes `ranges::subrange`, so
+// you most likely want to use `__pair_like_no_subrange` if you're looking for `pair-like`.
 
 #endif // _LIBCPP_STD_VER >= 20
 

libcxx/include/__tuple/tuple_like.h

@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TUPLE_TUPLE_LIKE_NO_SUBRANGE_H
+#define _LIBCPP___TUPLE_TUPLE_LIKE_NO_SUBRANGE_H
+
+#include <__config>
+#include <__fwd/array.h>
+#include <__fwd/complex.h>
+#include <__fwd/pair.h>
+#include <__fwd/tuple.h>
+#include <__tuple/tuple_size.h>
+#include <__type_traits/remove_cvref.h>
+#include <cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Tp>
+inline constexpr bool __tuple_like_no_subrange_impl = false;
+
+template <class... _Tp>
+inline constexpr bool __tuple_like_no_subrange_impl<tuple<_Tp...>> = true;
+
+template <class _T1, class _T2>
+inline constexpr bool __tuple_like_no_subrange_impl<pair<_T1, _T2>> = true;
+
+template <class _Tp, size_t _Size>
+inline constexpr bool __tuple_like_no_subrange_impl<array<_Tp, _Size>> = true;
+
+#  if _LIBCPP_STD_VER >= 26
+
+template <class _Tp>
+inline constexpr bool __tuple_like_no_subrange_impl<complex<_Tp>> = true;
+
+#  endif
+
+template <class _Tp>
+concept __tuple_like_no_subrange = __tuple_like_no_subrange_impl<remove_cvref_t<_Tp>>;
+
+// This is equivalent to the exposition-only type trait `pair-like`, except that it is false for specializations of
+// `ranges::subrange`. This is more useful than the pair-like concept in the standard because every use of `pair-like`
+// excludes `ranges::subrange`.
+template <class _Tp>
+concept __pair_like_no_subrange = __tuple_like_no_subrange<_Tp> && tuple_size<remove_cvref_t<_Tp>>::value == 2;
+
+#endif // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TUPLE_TUPLE_LIKE_NO_SUBRANGE_H

libcxx/include/__tuple/tuple_like_no_subrange.h

@@ -15,12 +15,11 @@
 #include <__config>
 #include <__fwd/array.h>
 #include <__fwd/pair.h>
-#include <__fwd/subrange.h>
 #include <__fwd/tuple.h>
-#include <__tuple/pair_like.h>
 #include <__tuple/sfinae_helpers.h>
 #include <__tuple/tuple_element.h>
 #include <__tuple/tuple_indices.h>
+#include <__tuple/tuple_like_no_subrange.h>
 #include <__tuple/tuple_size.h>
 #include <__type_traits/common_reference.h>
 #include <__type_traits/common_type.h>
@@ -60,14 +59,6 @@ struct __non_trivially_copyable_base {
   __non_trivially_copyable_base(__non_trivially_copyable_base const&) _NOEXCEPT {}
 };
 
-#if _LIBCPP_STD_VER >= 23
-template <class _Tp>
-struct __is_specialization_of_subrange : false_type {};
-
-template <class _Iter, class _Sent, ranges::subrange_kind _Kind>
-struct __is_specialization_of_subrange<ranges::subrange<_Iter, _Sent, _Kind>> : true_type {};
-#endif
-
 template <class _T1, class _T2>
 struct _LIBCPP_TEMPLATE_VIS pair
 #if defined(_LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR)
@@ -201,19 +192,19 @@ struct _LIBCPP_TEMPLATE_VIS pair
 #  endif
 
 #  if _LIBCPP_STD_VER >= 23
+  // TODO: Remove this workaround in LLVM 20. The bug got fixed in Clang 18.
   // This is a workaround for http://llvm.org/PR60710. We should be able to remove it once Clang is fixed.
   template <class _PairLike>
   _LIBCPP_HIDE_FROM_ABI static constexpr bool __pair_like_explicit_wknd() {
-    if constexpr (__pair_like<_PairLike>) {
+    if constexpr (__pair_like_no_subrange<_PairLike>) {
       return !is_convertible_v<decltype(std::get<0>(std::declval<_PairLike&&>())), first_type> ||
              !is_convertible_v<decltype(std::get<1>(std::declval<_PairLike&&>())), second_type>;
     }
     return false;
   }
 
-  template <__pair_like _PairLike>
-    requires(!__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value &&
-             is_constructible_v<first_type, decltype(std::get<0>(std::declval<_PairLike &&>()))> &&
+  template <__pair_like_no_subrange _PairLike>
+    requires(is_constructible_v<first_type, decltype(std::get<0>(std::declval<_PairLike &&>()))> &&
              is_constructible_v<second_type, decltype(std::get<1>(std::declval<_PairLike &&>()))>)
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(__pair_like_explicit_wknd<_PairLike>()) pair(_PairLike&& __p)
       : first(std::get<0>(std::forward<_PairLike>(__p))), second(std::get<1>(std::forward<_PairLike>(__p))) {}
@@ -306,8 +297,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     return *this;
   }
 
-  template <__pair_like _PairLike>
-    requires(__different_from<_PairLike, pair> && !__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value &&
+  template <__pair_like_no_subrange _PairLike>
+    requires(__different_from<_PairLike, pair> &&
              is_assignable_v<first_type&, decltype(std::get<0>(std::declval<_PairLike>()))> &&
              is_assignable_v<second_type&, decltype(std::get<1>(std::declval<_PairLike>()))>)
   _LIBCPP_HIDE_FROM_ABI constexpr pair& operator=(_PairLike&& __p) {
@@ -316,8 +307,8 @@ struct _LIBCPP_TEMPLATE_VIS pair
     return *this;
   }
 
-  template <__pair_like _PairLike>
-    requires(__different_from<_PairLike, pair> && !__is_specialization_of_subrange<remove_cvref_t<_PairLike>>::value &&
+  template <__pair_like_no_subrange _PairLike>
+    requires(__different_from<_PairLike, pair> &&
              is_assignable_v<first_type const&, decltype(std::get<0>(std::declval<_PairLike>()))> &&
              is_assignable_v<second_type const&, decltype(std::get<1>(std::declval<_PairLike>()))>)
   _LIBCPP_HIDE_FROM_ABI constexpr pair const& operator=(_PairLike&& __p) const {

libcxx/include/__utility/pair.h

@@ -705,12 +705,12 @@
   { include: [ "<__thread/timed_backoff_policy.h>", "private", "<thread>", "public" ] },
   { include: [ "<__tuple/find_index.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/make_tuple_types.h>", "private", "<tuple>", "public" ] },
-  { include: [ "<__tuple/pair_like.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/sfinae_helpers.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_element.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_indices.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_like.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_like_ext.h>", "private", "<tuple>", "public" ] },
+  { include: [ "<__tuple/tuple_like_no_subrange.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_size.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__tuple/tuple_types.h>", "private", "<tuple>", "public" ] },
   { include: [ "<__type_traits/add_const.h>", "private", "<type_traits>", "public" ] },

libcxx/include/libcxx.imp

@@ -1810,20 +1810,22 @@ module std_private_thread_thread               [system] {
 }
 module std_private_thread_timed_backoff_policy [system] { header "__thread/timed_backoff_policy.h" }
 
-module std_private_tuple_find_index       [system] { header "__tuple/find_index.h" }
-module std_private_tuple_make_tuple_types [system] { header "__tuple/make_tuple_types.h" }
-module std_private_tuple_pair_like        [system] {
-  header "__tuple/pair_like.h"
-  export std_private_tuple_tuple_like
-}
-module std_private_tuple_sfinae_helpers   [system] { header "__tuple/sfinae_helpers.h" }
-module std_private_tuple_tuple_element    [system] { header "__tuple/tuple_element.h" }
-module std_private_tuple_tuple_fwd        [system] { header "__fwd/tuple.h" }
-module std_private_tuple_tuple_indices    [system] { header "__tuple/tuple_indices.h" }
-module std_private_tuple_tuple_like       [system] { header "__tuple/tuple_like.h" }
-module std_private_tuple_tuple_like_ext   [system] { header "__tuple/tuple_like_ext.h" }
-module std_private_tuple_tuple_size       [system] { header "__tuple/tuple_size.h" }
-module std_private_tuple_tuple_types      [system] { header "__tuple/tuple_types.h" }
+module std_private_tuple_find_index             [system] { header "__tuple/find_index.h" }
+module std_private_tuple_make_tuple_types       [system] { header "__tuple/make_tuple_types.h" }
+module std_private_tuple_tuple_like_no_subrange [system] {
+  header "__tuple/tuple_like_no_subrange.h"
+}
+module std_private_tuple_sfinae_helpers         [system] { header "__tuple/sfinae_helpers.h" }
+module std_private_tuple_tuple_element          [system] { header "__tuple/tuple_element.h" }
+module std_private_tuple_tuple_fwd              [system] { header "__fwd/tuple.h" }
+module std_private_tuple_tuple_indices          [system] { header "__tuple/tuple_indices.h" }
+module std_private_tuple_tuple_like             [system] {
+  header "__tuple/tuple_like.h"
+  export *
+}
+module std_private_tuple_tuple_like_ext         [system] { header "__tuple/tuple_like_ext.h" }
+module std_private_tuple_tuple_size             [system] { header "__tuple/tuple_size.h" }
+module std_private_tuple_tuple_types            [system] { header "__tuple/tuple_types.h" }
 
 module std_private_type_traits_add_const                                 [system] { header "__type_traits/add_const.h" }
 module std_private_type_traits_add_cv                                    [system] { header "__type_traits/add_cv.h" }

libcxx/include/module.modulemap

@@ -235,11 +235,13 @@ namespace std {
 #include <__type_traits/is_destructible.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_reference.h>
 #include <__type_traits/is_trivially_assignable.h>
 #include <__type_traits/is_trivially_constructible.h>
 #include <__type_traits/is_trivially_destructible.h>
 #include <__type_traits/is_void.h>
 #include <__type_traits/remove_const.h>
+#include <__type_traits/remove_cvref.h>
 #include <__type_traits/type_identity.h>
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>

libcxx/include/variant

@@ -15,6 +15,7 @@
 // template<class T>
 //  concept tuple-like;           // exposition only
 
+#include <__tuple/tuple_like.h>
 #include <array>
 #include <complex>
 #include <ranges>

libcxx/test/libcxx/utilities/tuple/__tuple_like.compile.pass.cpp

@@ -52,7 +52,6 @@ static_assert(!std::ranges::input_range<Range<cpp20_output_iterator<std::tuple<i
 static_assert(!HasElementsView<Range<cpp20_output_iterator<std::tuple<int>*>>, 0>);
 
 // !tuple-like
-LIBCPP_STATIC_ASSERT(!std::__tuple_like<int>);
 static_assert(!HasElementsView<Range<int*>, 1>);
 
 // !(N < tuple_size_v<T>)