libstdc++: add support for constexpr stable_sort (P2562R1)

stable_sort has been made constexpr in C++26. Apart from plastering a
few functions with constexpr, there's an implementation challenge, that
is: stable_sort takes different codepaths in case extra memory can be
allocated. Rather than doing some major refactorings, simply use the
non-allocating path during constant evaluation. That's the same codepath
used when extra memory could not be allocated, as well as by
freestanding.

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h (stable_sort): Add constexpr.
	* include/bits/ranges_algo.h (__stable_sort_fn): Add constexpr
	to the function call operators.
	* include/bits/stl_algo.h (__stable_sort): Add constexpr.
	During constant evaluation, always use the non-allocating path.
	(stable_sort): Add constexpr.
	(__inplace_stable_sort): Likewise.
	(__merge_without_buffer): Likewise.
	* include/bits/version.def (constexpr_algorithms): Bump value
	for C++26.
	* include/bits/version.h: Regnerate.
	* testsuite/25_algorithms/cpp_lib_constexpr.cc: Test the bumped
	feature-testing macro.
	* testsuite/25_algorithms/headers/algorithm/synopsis.cc: Adapt
	the test to constexpr stable_sort.
	* testsuite/25_algorithms/stable_sort/constexpr.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

libstdc++-v3/include/bits/algorithmfwd.h

@@ -945,10 +945,12 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     sort(_RAIter, _RAIter, _Compare);
 
   template<typename _RAIter>
+    _GLIBCXX26_CONSTEXPR
     void
     stable_sort(_RAIter, _RAIter);
 
   template<typename _RAIter, typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     void
     stable_sort(_RAIter, _RAIter, _Compare);
 

libstdc++-v3/include/bits/ranges_algo.h

@@ -1842,6 +1842,7 @@ namespace ranges
     template<random_access_iterator _Iter, sentinel_for<_Iter> _Sent,
 	     typename _Comp = ranges::less, typename _Proj = identity>
       requires sortable<_Iter, _Comp, _Proj>
+      _GLIBCXX26_CONSTEXPR
       _Iter
       operator()(_Iter __first, _Sent __last,
 		 _Comp __comp = {}, _Proj __proj = {}) const
@@ -1855,6 +1856,7 @@ namespace ranges
     template<random_access_range _Range,
 	     typename _Comp = ranges::less, typename _Proj = identity>
       requires sortable<iterator_t<_Range>, _Comp, _Proj>
+      _GLIBCXX26_CONSTEXPR
       borrowed_iterator_t<_Range>
       operator()(_Range&& __r, _Comp __comp = {}, _Proj __proj = {}) const
       {

libstdc++-v3/include/bits/stl_algo.h

@@ -2415,6 +2415,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
   /// This is a helper function for the merge routines.
   template<typename _BidirectionalIterator, typename _Distance,
 	   typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     void
     __merge_without_buffer(_BidirectionalIterator __first,
 			   _BidirectionalIterator __middle,
@@ -2723,6 +2724,7 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
 
   /// This is a helper function for the stable sorting routines.
   template<typename _RandomAccessIterator, typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     void
     __inplace_stable_sort(_RandomAccessIterator __first,
 			  _RandomAccessIterator __last, _Compare __comp)
@@ -4971,6 +4973,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     }
 
   template<typename _RandomAccessIterator, typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     inline void
     __stable_sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
 		  _Compare __comp)
@@ -4984,6 +4987,12 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 	return;
 
 #if _GLIBCXX_HOSTED
+      if (__is_constant_evaluated())
+	{
+	  std::__inplace_stable_sort(__first, __last, __comp);
+	  return;
+	}
+
       typedef _Temporary_buffer<_RandomAccessIterator, _ValueType> _TmpBuf;
       // __stable_sort_adaptive sorts the range in two halves,
       // so the buffer only needs to fit half the range at once.
@@ -5021,6 +5030,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  ordering after calling @p stable_sort().
   */
   template<typename _RandomAccessIterator>
+    _GLIBCXX26_CONSTEXPR
     inline void
     stable_sort(_RandomAccessIterator __first, _RandomAccessIterator __last)
     {
@@ -5055,6 +5065,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  relative ordering after calling @p stable_sort().
   */
   template<typename _RandomAccessIterator, typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     inline void
     stable_sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
 		_Compare __comp)

libstdc++-v3/include/bits/version.def

@@ -1110,6 +1110,10 @@ ftms = {
 
 ftms = {
   name = constexpr_algorithms;
+  values = {
+    v = 202306;
+    cxxmin = 26;
+  };
   values = {
     v = 201806;
     cxxmin = 20;

libstdc++-v3/include/bits/version.h

@@ -1251,7 +1251,12 @@
 #undef __glibcxx_want_constexpr_functional
 
 #if !defined(__cpp_lib_constexpr_algorithms)
-# if (__cplusplus >= 202002L)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_constexpr_algorithms 202306L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_algorithms)
+#   define __cpp_lib_constexpr_algorithms 202306L
+#  endif
+# elif (__cplusplus >= 202002L)
 #  define __glibcxx_constexpr_algorithms 201806L
 #  if defined(__glibcxx_want_all) || defined(__glibcxx_want_constexpr_algorithms)
 #   define __cpp_lib_constexpr_algorithms 201806L

libstdc++-v3/testsuite/25_algorithms/cpp_lib_constexpr.cc

@@ -22,6 +22,10 @@
 
 #ifndef __cpp_lib_constexpr_algorithms
 # error "Feature-test macro for constexpr algorithms missing"
+#elif __cplusplus > 202302L
+# if __cpp_lib_constexpr_algorithms < 202306L
+#  error "Feature-test macro for constexpr algorithms has wrong value"
+# endif
 #elif __cpp_lib_constexpr_algorithms < 201806L
 # error "Feature-test macro for constexpr algorithms has wrong value"
 #endif

libstdc++-v3/testsuite/25_algorithms/headers/algorithm/synopsis.cc

@@ -365,10 +365,12 @@ namespace std
     sort(_RAIter, _RAIter, _Compare);
 
   template<typename _RAIter>
+    _GLIBCXX26_CONSTEXPR
     void
     stable_sort(_RAIter, _RAIter);
 
   template<typename _RAIter, typename _Compare>
+    _GLIBCXX26_CONSTEXPR
     void
     stable_sort(_RAIter, _RAIter, _Compare);
 

libstdc++-v3/testsuite/25_algorithms/stable_sort/constexpr.cc

@@ -0,0 +1,62 @@
+// { dg-do compile { target c++26 } }
+
+#include <algorithm>
+#include <array>
+#include <functional>
+
+constexpr auto
+createArray()
+{
+  return std::to_array({10, 0, 1, 2, 5, 6, 7, 8, 3, 4, 9, 11});
+}
+
+constexpr bool
+test01()
+{
+  auto ar = createArray();
+  std::stable_sort(ar.begin(), ar.end());
+  return std::is_sorted(ar.begin(), ar.end());
+}
+
+static_assert(test01());
+
+constexpr bool
+test02()
+{
+  auto ar = createArray();
+  std::stable_sort(ar.begin(), ar.end(), std::greater<>());
+  return std::is_sorted(ar.begin(), ar.end(), std::greater<>());
+}
+
+static_assert(test02());
+
+constexpr bool
+test03()
+{
+  auto ar = createArray();
+  std::ranges::stable_sort(ar);
+  return std::ranges::is_sorted(ar);
+}
+
+static_assert(test03());
+
+constexpr bool
+test04()
+{
+  auto ar = createArray();
+  std::ranges::stable_sort(ar, std::ranges::greater());
+  return std::ranges::is_sorted(ar, std::ranges::greater());
+}
+
+static_assert(test04());
+
+constexpr bool
+test05()
+{
+  auto ar = createArray();
+  auto proj = [](int i) { return -i; };
+  std::ranges::stable_sort(ar, {}, proj);
+  return std::ranges::is_sorted(ar, {}, proj);
+}
+
+static_assert(test05());