[libc++] Implement part of P2562R1: constexpr `ranges::inplace_merge` (#131947)
Drive-by changes:
- Consistently mark `std::__inplace_merge::__inplace_merge_impl`
`_LIBCPP_CONSTEXPR_SINCE_CXX26`.
- This function template is only called by other functions that becomes
constexpr since C++26, and it itself calls `std::__inplace_merge` that
is constexpr since C++26.
- Unblock related test coverage in constant evaluation for
`stable_partition`, `ranges::stable_sort`, `std::stable_sort`,
`std::stable_partition`, and `std::inplace_merge`.
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index aea24e5..6ba903a 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1031,13 +1031,14 @@
template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
class Proj = identity>
requires sortable<I, Comp, Proj>
- I inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {}); // since C++20
+ constexpr I // constexpr since C++26
+ inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {}); // since C++20
template<bidirectional_range R, class Comp = ranges::less, class Proj = identity>
requires sortable<iterator_t<R>, Comp, Proj>
- borrowed_iterator_t<R>
+ constexpr borrowed_iterator_t<R> // constexpr since C++26
inplace_merge(R&& r, iterator_t<R> middle, Comp comp = {},
- Proj proj = {}); // since C++20
+ Proj proj = {}); // since C++20
template<permutable I, sentinel_for<I> S, class Proj = identity,
indirect_equivalence_relation<projected<I, Proj>> C = ranges::equal_to>
