Hello,

as usual thank you for the review. V2 is attached.

On 02/08/2024 14:38, Jonathan Wakely wrote:
On Fri, 2 Aug 2024 at 13:17, Jonathan Wakely <jwak...@redhat.com> wrote:

On Fri, 2 Aug 2024 at 11:45, Giuseppe D'Angelo wrote:

Hello,

The attached patch adds support for P2248R8 + P3217R0 (Enabling
list-initialization for algorithms, C++26). The big question is whether
this keeps the code readable enough without introducing too much
#ifdef-ery, so any feedback is appreciated.

Putting the extra args on the algorithmfwd.h declarations is a nice
way to avoid any clutter on the definitions. I think that is very
readable.
Another option would be to not touch those forward declarations, but
add new ones with the defaults:

#if __glibcxx_default_template_type_for_algorithm_values
// new declarations with default template args ...
#endif

But I think what you've done is good.

I'll keep it then :)

For ranges_algo.h I'm almost tempted to say we should just treat this
as a DR, to avoid the #ifdef-ery. Almost.
Is there any reason we can't rearrange the template parameters fo
C++20 and C++23 mode? I don't think users are allowed to use explicit
template argument lists for invoke e.g. ranges::find.operator()<Iter,
Proj> so it should be unobservable if we change the order for C++20
(and just don't add the default args until C++26). That might reduce
the differences to just a line or two for each CPO.

Indeed, users cannot rely on any specific order of the template arguments when calling algorithms. This is

https://eel.is/c++draft/algorithms.requirements#15

which has this note:

"Consequently, an implementation can declare an algorithm with different template parameters than those presented"

which of course does apply here: it's why P2248 could do these changes to begin with. The only reason why I kept them in the original order was a matter of caution, but sure, in the new patch I've changed them unconditionally and just used a macro to hide the default in pre-C++26 modes. This should keep the code clean(er).


The merged wording also removes the redundant 'typename' from the
default arguments, but I think we might still need that for Clang
compat. I'm not sure when Clang fully implemented "down with
typename", but it was still causing issues some time last year.

I hope it's fine if I keep it.

Thanks,
--
Giuseppe D'Angelo

From 607c43341502b7f6e34a24e9f2ecbdaae504cf4e Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
Date: Fri, 2 Aug 2024 00:23:04 +0200
Subject: [PATCH] libstdc++: add default template parameters to algorithms

This implements P2248R8 + P3217R0, both approved for C++26.
The changes are mostly mechanical; the struggle is to keep readability
with the pre-P2248 signatures.

* For containers, "classic STL" algorithms and their parallel versions,
  introduce a macro and amend their declarations/definitions with it.
  The macro either expands to the defaulted parameter or to nothing
  in pre-C++26 modes.

* For range algorithms, we need to reorder their template parameters.
  I've done so unconditionally, because users cannot rely on template
  parameters of algorithms (this is explicitly authorized by
  [algorithms.requirements]/15). The defaults are then hidden behind
  another macro.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h: Add projected_value_t.
	* include/bits/algorithmfwd.h: Add the default template
	parameter to the relevant forward declarations.
	* include/pstl/glue_algorithm_defs.h: Likewise.
	* include/bits/ranges_algo.h: Add the default template
	parameter to range-based algorithms.
	* include/bits/ranges_algobase.h: Likewise.
	* include/bits/ranges_util.h: Likewise.
        * include/bits/stl_iterator_base_types.h: Add helper macro.
	* include/bits/version.def: Add the new feature-testing macro.
	* include/bits/version.h: Regenerate.
	* include/std/algorithm: Pull the feature-testing macro.
	* include/std/ranges: Likewise.
	* include/std/deque: Pull the feature-testing macro, add
	the default for std::erase.
	* include/std/forward_list: Likewise.
	* include/std/list: Likewise.
	* include/std/string: Likewise.
	* include/std/vector: Likewise.
	* testsuite/23_containers/default_template_value.cc: New test.
	* testsuite/25_algorithms/default_template_value.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dang...@kdab.com>
---
 libstdc++-v3/include/bits/algorithmfwd.h      |  61 ++++---
 libstdc++-v3/include/bits/iterator_concepts.h |   7 +
 libstdc++-v3/include/bits/ranges_algo.h       | 154 +++++++++++++-----
 libstdc++-v3/include/bits/ranges_algobase.h   |  16 +-
 libstdc++-v3/include/bits/ranges_base.h       |   9 +
 libstdc++-v3/include/bits/ranges_util.h       |  10 +-
 .../include/bits/stl_iterator_base_types.h    |   7 +
 libstdc++-v3/include/bits/version.def         |   8 +
 libstdc++-v3/include/bits/version.h           |  10 ++
 .../include/pstl/glue_algorithm_defs.h        |  23 +--
 libstdc++-v3/include/std/algorithm            |   1 +
 libstdc++-v3/include/std/deque                |   4 +-
 libstdc++-v3/include/std/forward_list         |   4 +-
 libstdc++-v3/include/std/list                 |   4 +-
 libstdc++-v3/include/std/ranges               |   1 +
 libstdc++-v3/include/std/string               |   4 +-
 libstdc++-v3/include/std/vector               |   4 +-
 .../23_containers/default_template_value.cc   |  40 +++++
 .../25_algorithms/default_template_value.cc   | 142 ++++++++++++++++
 19 files changed, 428 insertions(+), 81 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/default_template_value.cc
 create mode 100644 libstdc++-v3/testsuite/25_algorithms/default_template_value.cc

diff --git a/libstdc++-v3/include/bits/algorithmfwd.h b/libstdc++-v3/include/bits/algorithmfwd.h
index 34bf9921f43..88fac34e72f 100644
--- a/libstdc++-v3/include/bits/algorithmfwd.h
+++ b/libstdc++-v3/include/bits/algorithmfwd.h
@@ -203,12 +203,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     any_of(_IIter, _IIter, _Predicate);
 #endif
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     bool
     binary_search(_FIter, _FIter, const _Tp&);
 
-  template<typename _FIter, typename _Tp, typename _Compare>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter),
+	   typename _Compare>
     _GLIBCXX20_CONSTEXPR
     bool
     binary_search(_FIter, _FIter, const _Tp&, _Compare);
@@ -250,22 +253,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // count
   // count_if
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     pair<_FIter, _FIter>
     equal_range(_FIter, _FIter, const _Tp&);
 
-  template<typename _FIter, typename _Tp, typename _Compare>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter),
+	   typename _Compare>
     _GLIBCXX20_CONSTEXPR
     pair<_FIter, _FIter>
     equal_range(_FIter, _FIter, const _Tp&, _Compare);
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     void
     fill(_FIter, _FIter, const _Tp&);
 
-  template<typename _OIter, typename _Size, typename _Tp>
+  template<typename _OIter, typename _Size, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_OIter)>
     _GLIBCXX20_CONSTEXPR
     _OIter
     fill_n(_OIter, _Size, const _Tp&);
@@ -377,12 +385,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     void
     iter_swap(_FIter1, _FIter2);
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     _FIter
     lower_bound(_FIter, _FIter, const _Tp&);
 
-  template<typename _FIter, typename _Tp, typename _Compare>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter),
+	   typename _Compare>
     _GLIBCXX20_CONSTEXPR
     _FIter
     lower_bound(_FIter, _FIter, const _Tp&, _Compare);
@@ -553,7 +564,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // random_shuffle
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     _FIter
     remove(_FIter, _FIter, const _Tp&);
@@ -563,7 +575,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _FIter
     remove_if(_FIter, _FIter, _Predicate);
 
-  template<typename _IIter, typename _OIter, typename _Tp>
+  template<typename _IIter, typename _OIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_IIter)>
     _GLIBCXX20_CONSTEXPR
     _OIter
     remove_copy(_IIter, _IIter, _OIter, const _Tp&);
@@ -580,7 +593,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _OIter
     replace_copy(_IIter, _IIter, _OIter, const _Tp&, const _Tp&);
 
-  template<typename _Iter, typename _OIter, typename _Predicate, typename _Tp>
+  template<typename _Iter, typename _OIter, typename _Predicate, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_OIter)>
     _GLIBCXX20_CONSTEXPR
     _OIter
     replace_copy_if(_Iter, _Iter, _OIter, _Predicate, const _Tp&);
@@ -673,12 +687,15 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
 
   // unique_copy
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     _FIter
     upper_bound(_FIter, _FIter, const _Tp&);
 
-  template<typename _FIter, typename _Tp, typename _Compare>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter),
+	   typename _Compare>
     _GLIBCXX20_CONSTEXPR
     _FIter
     upper_bound(_FIter, _FIter, const _Tp&, _Compare);
@@ -695,7 +712,8 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     _FIter
     adjacent_find(_FIter, _FIter, _BinaryPredicate);
 
-  template<typename _IIter, typename _Tp>
+  template<typename _IIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_IIter)>
     _GLIBCXX20_CONSTEXPR
     typename iterator_traits<_IIter>::difference_type
     count(_IIter, _IIter, const _Tp&);
@@ -715,7 +733,8 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     bool
     equal(_IIter1, _IIter1, _IIter2, _BinaryPredicate);
 
-  template<typename _IIter, typename _Tp>
+  template<typename _IIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_IIter)>
     _GLIBCXX20_CONSTEXPR
     _IIter
     find(_IIter, _IIter, const _Tp&);
@@ -843,12 +862,14 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 #endif
 #endif // HOSTED
 
-  template<typename _FIter, typename _Tp>
+  template<typename _FIter, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     void
     replace(_FIter, _FIter, const _Tp&, const _Tp&);
 
-  template<typename _FIter, typename _Predicate, typename _Tp>
+  template<typename _FIter, typename _Predicate, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     void
     replace_if(_FIter, _FIter, _Predicate, const _Tp&);
@@ -863,12 +884,14 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     _FIter1
     search(_FIter1, _FIter1, _FIter2, _FIter2, _BinaryPredicate);
 
-  template<typename _FIter, typename _Size, typename _Tp>
+  template<typename _FIter, typename _Size, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter)>
     _GLIBCXX20_CONSTEXPR
     _FIter
     search_n(_FIter, _FIter, _Size, const _Tp&);
 
-  template<typename _FIter, typename _Size, typename _Tp,
+  template<typename _FIter, typename _Size, typename _Tp
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_FIter),
 	   typename _BinaryPredicate>
     _GLIBCXX20_CONSTEXPR
     _FIter
diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index ce0b8a10f88..4c5a9378d38 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -799,6 +799,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	   indirectly_regular_unary_invocable<_Iter> _Proj>
     using projected = typename __detail::__projected<_Iter, _Proj>::__type;
 
+#if __glibcxx_algorithm_default_value_type // C++ >= 26
+  template<indirectly_readable _Iter,
+	   indirectly_regular_unary_invocable<_Iter> _Proj>
+    using projected_value_t
+	= remove_cvref_t<invoke_result_t<_Proj&, iter_value_t<_Iter>&>>;
+#endif
+
   // [alg.req], common algorithm requirements
 
   /// [alg.req.ind.move], concept `indirectly_movable`
diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index d258be0b93f..9a117ae1ff1 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -281,7 +281,9 @@ namespace ranges
   struct __count_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity>
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<_Iter, _Proj>,
 					 const _Tp*>
@@ -296,7 +298,9 @@ namespace ranges
 	return __n;
       }
 
-    template<input_range _Range, typename _Tp, typename _Proj = identity>
+    template<input_range _Range, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<iterator_t<_Range>, _Proj>,
 					 const _Tp*>
@@ -344,8 +348,10 @@ namespace ranges
 
   struct __search_n_fn
   {
-    template<forward_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp,
-	     typename _Pred = ranges::equal_to, typename _Proj = identity>
+    template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
+	     typename _Pred = ranges::equal_to, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirectly_comparable<_Iter, const _Tp*, _Pred, _Proj>
       constexpr subrange<_Iter>
       operator()(_Iter __first, _Sent __last, iter_difference_t<_Iter> __count,
@@ -416,8 +422,10 @@ namespace ranges
 	  }
       }
 
-    template<forward_range _Range, typename _Tp,
-	     typename _Pred = ranges::equal_to, typename _Proj = identity>
+    template<forward_range _Range,
+	     typename _Pred = ranges::equal_to, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirectly_comparable<iterator_t<_Range>, const _Tp*,
 				     _Pred, _Proj>
       constexpr borrowed_subrange_t<_Range>
@@ -774,7 +782,11 @@ namespace ranges
   struct __replace_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp1, typename _Tp2, typename _Proj = identity>
+	     typename _Proj = identity,
+	     typename _Tp1
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
+	     typename _Tp2
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp1)>
       requires indirectly_writable<_Iter, const _Tp2&>
 	&& indirect_binary_predicate<ranges::equal_to, projected<_Iter, _Proj>,
 				     const _Tp1*>
@@ -789,8 +801,11 @@ namespace ranges
 	return __first;
       }
 
-    template<input_range _Range,
-	     typename _Tp1, typename _Tp2, typename _Proj = identity>
+    template<input_range _Range, typename _Proj = identity,
+	     typename _Tp1
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
+	     typename _Tp2
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp1)>
       requires indirectly_writable<iterator_t<_Range>, const _Tp2&>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<iterator_t<_Range>, _Proj>,
@@ -810,7 +825,9 @@ namespace ranges
   struct __replace_if_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
 	     indirect_unary_predicate<projected<_Iter, _Proj>> _Pred>
       requires indirectly_writable<_Iter, const _Tp&>
       constexpr _Iter
@@ -823,7 +840,9 @@ namespace ranges
 	return std::move(__first);
       }
 
-    template<input_range _Range, typename _Tp, typename _Proj = identity,
+    template<input_range _Range, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
 	     indirect_unary_predicate<projected<iterator_t<_Range>, _Proj>>
 	       _Pred>
       requires indirectly_writable<iterator_t<_Range>, const _Tp&>
@@ -844,11 +863,15 @@ namespace ranges
   struct __replace_copy_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp1, typename _Tp2, output_iterator<const _Tp2&> _Out,
-	     typename _Proj = identity>
+	     typename _Out, typename _Proj = identity,
+	     typename _Tp1
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
+	     typename _Tp2
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>)>
       requires indirectly_copyable<_Iter, _Out>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<_Iter, _Proj>, const _Tp1*>
+	&& output_iterator<_Out, const _Tp2&>
       constexpr replace_copy_result<_Iter, _Out>
       operator()(_Iter __first, _Sent __last, _Out __result,
 		 const _Tp1& __old_value, const _Tp2& __new_value,
@@ -862,12 +885,17 @@ namespace ranges
 	return {std::move(__first), std::move(__result)};
       }
 
-    template<input_range _Range, typename _Tp1, typename _Tp2,
-	     output_iterator<const _Tp2&> _Out, typename _Proj = identity>
+    template<input_range _Range, typename _Out,
+	     typename _Proj = identity,
+	     typename _Tp1
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
+	     typename _Tp2
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>)>
       requires indirectly_copyable<iterator_t<_Range>, _Out>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<iterator_t<_Range>, _Proj>,
 				     const _Tp1*>
+	&& output_iterator<_Out, const _Tp2&>
       constexpr replace_copy_result<borrowed_iterator_t<_Range>, _Out>
       operator()(_Range&& __r, _Out __result,
 		 const _Tp1& __old_value, const _Tp2& __new_value,
@@ -887,10 +915,13 @@ namespace ranges
   struct __replace_copy_if_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, output_iterator<const _Tp&> _Out,
+	     typename _Out,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>),
 	     typename _Proj = identity,
 	     indirect_unary_predicate<projected<_Iter, _Proj>> _Pred>
       requires indirectly_copyable<_Iter, _Out>
+	&& output_iterator<_Out, const _Tp&>
       constexpr replace_copy_if_result<_Iter, _Out>
       operator()(_Iter __first, _Sent __last, _Out __result,
 		 _Pred __pred, const _Tp& __new_value, _Proj __proj = {}) const
@@ -904,11 +935,14 @@ namespace ranges
       }
 
     template<input_range _Range,
-	     typename _Tp, output_iterator<const _Tp&> _Out,
+	     typename _Out,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>),
 	     typename _Proj = identity,
 	     indirect_unary_predicate<projected<iterator_t<_Range>, _Proj>>
 	       _Pred>
       requires indirectly_copyable<iterator_t<_Range>, _Out>
+	&& output_iterator<_Out, const _Tp&>
       constexpr replace_copy_if_result<borrowed_iterator_t<_Range>, _Out>
       operator()(_Range&& __r, _Out __result,
 		 _Pred __pred, const _Tp& __new_value, _Proj __proj = {}) const
@@ -1004,7 +1038,9 @@ namespace ranges
   struct __remove_fn
   {
     template<permutable _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity>
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<_Iter, _Proj>,
 					 const _Tp*>
@@ -1019,7 +1055,9 @@ namespace ranges
 				 std::move(__pred), std::move(__proj));
       }
 
-    template<forward_range _Range, typename _Tp, typename _Proj = identity>
+    template<forward_range _Range, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires permutable<iterator_t<_Range>>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<iterator_t<_Range>, _Proj>,
@@ -1079,7 +1117,9 @@ namespace ranges
   struct __remove_copy_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     weakly_incrementable _Out, typename _Tp, typename _Proj = identity>
+	     weakly_incrementable _Out, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirectly_copyable<_Iter, _Out>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<_Iter, _Proj>,
@@ -1098,7 +1138,9 @@ namespace ranges
       }
 
     template<input_range _Range, weakly_incrementable _Out,
-	     typename _Tp, typename _Proj = identity>
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirectly_copyable<iterator_t<_Range>, _Out>
 	&& indirect_binary_predicate<ranges::equal_to,
 				     projected<iterator_t<_Range>, _Proj>,
@@ -2047,7 +2089,9 @@ namespace ranges
   struct __lower_bound_fn
   {
     template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
 	     indirect_strict_weak_order<const _Tp*, projected<_Iter, _Proj>>
 	       _Comp = ranges::less>
       constexpr _Iter
@@ -2073,7 +2117,10 @@ namespace ranges
 	return __first;
       }
 
-    template<forward_range _Range, typename _Tp, typename _Proj = identity,
+    template<forward_range _Range,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
 	     indirect_strict_weak_order<const _Tp*,
 					projected<iterator_t<_Range>, _Proj>>
 	       _Comp = ranges::less>
@@ -2091,7 +2138,9 @@ namespace ranges
   struct __upper_bound_fn
   {
     template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
 	     indirect_strict_weak_order<const _Tp*, projected<_Iter, _Proj>>
 	       _Comp = ranges::less>
       constexpr _Iter
@@ -2117,7 +2166,10 @@ namespace ranges
 	return __first;
       }
 
-    template<forward_range _Range, typename _Tp, typename _Proj = identity,
+    template<forward_range _Range,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
 	     indirect_strict_weak_order<const _Tp*,
 					projected<iterator_t<_Range>, _Proj>>
 	       _Comp = ranges::less>
@@ -2135,7 +2187,9 @@ namespace ranges
   struct __equal_range_fn
   {
     template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
 	     indirect_strict_weak_order<const _Tp*, projected<_Iter, _Proj>>
 	       _Comp = ranges::less>
       constexpr subrange<_Iter>
@@ -2177,7 +2231,9 @@ namespace ranges
       }
 
     template<forward_range _Range,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
 	     indirect_strict_weak_order<const _Tp*,
 					projected<iterator_t<_Range>, _Proj>>
 	       _Comp = ranges::less>
@@ -2195,7 +2251,9 @@ namespace ranges
   struct __binary_search_fn
   {
     template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj),
 	     indirect_strict_weak_order<const _Tp*, projected<_Iter, _Proj>>
 	       _Comp = ranges::less>
       constexpr bool
@@ -2210,7 +2268,9 @@ namespace ranges
       }
 
     template<forward_range _Range,
-	     typename _Tp, typename _Proj = identity,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj),
 	     indirect_strict_weak_order<const _Tp*,
 					projected<iterator_t<_Range>, _Proj>>
 	       _Comp = ranges::less>
@@ -3469,14 +3529,19 @@ namespace ranges
   struct __contains_fn
   {
     template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
-	    typename _Tp, typename _Proj = identity>
+	    typename _Proj = identity,
+	    typename _Tp
+	      _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<_Iter, _Proj>, const _Tp*>
       constexpr bool
       operator()(_Iter __first, _Sent __last, const _Tp& __value, _Proj __proj = {}) const
       { return ranges::find(std::move(__first), __last, __value, std::move(__proj)) != __last; }
 
-    template<input_range _Range, typename _Tp, typename _Proj = identity>
+    template<input_range _Range,
+	    typename _Proj = identity,
+	    typename _Tp
+	      _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<iterator_t<_Range>, _Proj>, const _Tp*>
       constexpr bool
@@ -3525,7 +3590,10 @@ namespace ranges
 
   struct __find_last_fn
   {
-    template<forward_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp, typename _Proj = identity>
+    template<forward_iterator _Iter, sentinel_for<_Iter> _Sent,
+	     typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to, projected<_Iter, _Proj>, const _Tp*>
       constexpr subrange<_Iter>
       operator()(_Iter __first, _Sent __last, const _Tp& __value, _Proj __proj = {}) const
@@ -3556,7 +3624,9 @@ namespace ranges
 	  }
       }
 
-    template<forward_range _Range, typename _Tp, typename _Proj = identity>
+    template<forward_range _Range, typename _Proj = identity,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<_Range>, _Proj>, const _Tp*>
       constexpr borrowed_subrange_t<_Range>
       operator()(_Range&& __r, const _Tp& __value, _Proj __proj = {}) const
@@ -3730,7 +3800,8 @@ namespace ranges
 	return _Ret{std::move(__first), std::move(__accum)};
       }
 
-    template<input_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp,
+    template<input_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Iter>),
 	     __detail::__indirectly_binary_left_foldable<_Tp, _Iter> _Fp>
       constexpr auto
       operator()(_Iter __first, _Sent __last, _Tp __init, _Fp __f) const
@@ -3740,7 +3811,8 @@ namespace ranges
 				  std::move(__init), std::move(__f));
       }
 
-    template<input_range _Range, typename _Tp,
+    template<input_range _Range, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(range_value_t<_Range>),
 	     __detail::__indirectly_binary_left_foldable<_Tp, iterator_t<_Range>> _Fp>
       constexpr auto
       operator()(_Range&& __r, _Tp __init, _Fp __f) const
@@ -3755,7 +3827,8 @@ namespace ranges
 
   struct __fold_left_fn
   {
-    template<input_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp,
+    template<input_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Iter>),
 	     __detail::__indirectly_binary_left_foldable<_Tp, _Iter> _Fp>
       constexpr auto
       operator()(_Iter __first, _Sent __last, _Tp __init, _Fp __f) const
@@ -3764,7 +3837,8 @@ namespace ranges
 					   std::move(__init), std::move(__f)).value;
       }
 
-    template<input_range _Range, typename _Tp,
+    template<input_range _Range, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(range_value_t<_Range>),
 	     __detail::__indirectly_binary_left_foldable<_Tp, iterator_t<_Range>> _Fp>
       constexpr auto
       operator()(_Range&& __r, _Tp __init, _Fp __f) const
@@ -3842,7 +3916,8 @@ namespace ranges
 
   struct __fold_right_fn
   {
-    template<bidirectional_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp,
+    template<bidirectional_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Iter>),
 	     __detail::__indirectly_binary_right_foldable<_Tp, _Iter> _Fp>
       constexpr auto
       operator()(_Iter __first, _Sent __last, _Tp __init, _Fp __f) const
@@ -3859,7 +3934,8 @@ namespace ranges
 	return __accum;
       }
 
-    template<bidirectional_range _Range, typename _Tp,
+    template<bidirectional_range _Range, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(range_value_t<_Range>),
 	     __detail::__indirectly_binary_right_foldable<_Tp, iterator_t<_Range>> _Fp>
       constexpr auto
       operator()(_Range&& __r, _Tp __init, _Fp __f) const
diff --git a/libstdc++-v3/include/bits/ranges_algobase.h b/libstdc++-v3/include/bits/ranges_algobase.h
index fd35b8ba14c..39cf06bdd3e 100644
--- a/libstdc++-v3/include/bits/ranges_algobase.h
+++ b/libstdc++-v3/include/bits/ranges_algobase.h
@@ -536,7 +536,9 @@ namespace ranges
 
   struct __fill_n_fn
   {
-    template<typename _Tp, output_iterator<const _Tp&> _Out>
+    template<typename _Out, typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>)>
+      requires output_iterator<_Out, const _Tp&>
       constexpr _Out
       operator()(_Out __first, iter_difference_t<_Out> __n,
 		 const _Tp& __value) const
@@ -581,8 +583,11 @@ namespace ranges
 
   struct __fill_fn
   {
-    template<typename _Tp,
-	     output_iterator<const _Tp&> _Out, sentinel_for<_Out> _Sent>
+    template<typename _Out,
+	     sentinel_for<_Out> _Sent,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(iter_value_t<_Out>)>
+      requires output_iterator<_Out, const _Tp&>
       constexpr _Out
       operator()(_Out __first, _Sent __last, const _Tp& __value) const
       {
@@ -608,7 +613,10 @@ namespace ranges
 	  }
       }
 
-    template<typename _Tp, output_range<const _Tp&> _Range>
+    template<typename _Range,
+	     typename _Tp
+	       _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(range_value_t<_Range>)>
+      requires output_range<_Range, const _Tp&>
       constexpr borrowed_iterator_t<_Range>
       operator()(_Range&& __r, const _Tp& __value) const
       {
diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 63eb552b48b..1a26df4c4c3 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -39,6 +39,15 @@
 #include <bits/max_size_type.h>
 #include <bits/version.h>
 
+#if __glibcxx_algorithm_default_value_type // C++ >= 26
+# define _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(_X) = _X
+# define _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_I, _P) \
+     = projected_value_t<_I, _P>
+#else
+# define _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE(_X)
+# define _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_I, _P)
+#endif
+
 #ifdef __cpp_lib_concepts
 namespace std _GLIBCXX_VISIBILITY(default)
 {
diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index e6d96073e87..139c272046f 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -487,8 +487,10 @@ namespace ranges
 {
   struct __find_fn
   {
-    template<input_iterator _Iter, sentinel_for<_Iter> _Sent, typename _Tp,
-	     typename _Proj = identity>
+    template<input_iterator _Iter, sentinel_for<_Iter> _Sent,
+	     typename _Proj = identity,
+	     typename _Tp
+	     _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(_Iter, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<_Iter, _Proj>, const _Tp*>
       constexpr _Iter
@@ -521,7 +523,9 @@ namespace ranges
 	return __first;
       }
 
-    template<input_range _Range, typename _Tp, typename _Proj = identity>
+    template<input_range _Range, typename _Proj = identity,
+	     typename _Tp
+	     _GLIBCXX26_RANGE_ALGORITHM_DEFAULT_VALUE_TYPE_PROJ(iterator_t<_Range>, _Proj)>
       requires indirect_binary_predicate<ranges::equal_to,
 					 projected<iterator_t<_Range>, _Proj>,
 					 const _Tp*>
diff --git a/libstdc++-v3/include/bits/stl_iterator_base_types.h b/libstdc++-v3/include/bits/stl_iterator_base_types.h
index 07beec3554d..2b87324eb77 100644
--- a/libstdc++-v3/include/bits/stl_iterator_base_types.h
+++ b/libstdc++-v3/include/bits/stl_iterator_base_types.h
@@ -269,4 +269,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 
+#if __glibcxx_algorithm_default_value_type // C++ >= 26
+# define _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_Iterator) \
+     = typename iterator_traits<_Iterator>::value_type
+#else
+# define _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_It)
+#endif
+
 #endif /* _STL_ITERATOR_BASE_TYPES_H */
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 47335bfa6dd..08dfd51c9db 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1828,6 +1828,14 @@ ftms = {
   };
 };
 
+ftms = {
+  name = algorithm_default_value_type;
+  values = {
+    v = 202403;
+    cxxmin = 26;
+  };
+};
+
 // Standard test specifications.
 stds[97] = ">= 199711L";
 stds[03] = ">= 199711L";
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index defe54ac1f2..efd2c33aab7 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -2038,4 +2038,14 @@
 #endif /* !defined(__cpp_lib_is_virtual_base_of) && defined(__glibcxx_want_is_virtual_base_of) */
 #undef __glibcxx_want_is_virtual_base_of
 
+#if !defined(__cpp_lib_algorithm_default_value_type)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_algorithm_default_value_type 202403L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_algorithm_default_value_type)
+#   define __cpp_lib_algorithm_default_value_type 202403L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_algorithm_default_value_type) && defined(__glibcxx_want_algorithm_default_value_type) */
+#undef __glibcxx_want_algorithm_default_value_type
+
 #undef __glibcxx_want_all
diff --git a/libstdc++-v3/include/pstl/glue_algorithm_defs.h b/libstdc++-v3/include/pstl/glue_algorithm_defs.h
index cef78e22e31..cf997b5f3bc 100644
--- a/libstdc++-v3/include/pstl/glue_algorithm_defs.h
+++ b/libstdc++-v3/include/pstl/glue_algorithm_defs.h
@@ -10,6 +10,7 @@
 #ifndef _PSTL_GLUE_ALGORITHM_DEFS_H
 #define _PSTL_GLUE_ALGORITHM_DEFS_H
 
+#include <bits/stl_iterator_base_types.h>
 #include <bits/stl_pair.h>
 
 #include "execution_defs.h"
@@ -55,7 +56,7 @@ template <class _ExecutionPolicy, class _ForwardIterator, class _Predicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 find_if_not(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Predicate __pred);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 find(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
 
@@ -95,7 +96,7 @@ adjacent_find(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardItera
 
 // [alg.count]
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy,
                                                  typename iterator_traits<_ForwardIterator>::difference_type>
 count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
@@ -117,12 +118,12 @@ __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardItera
 search(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __s_first,
        _ForwardIterator2 __s_last);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp, class _BinaryPredicate>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator), class _BinaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 search_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
          const _Tp& __value, _BinaryPredicate __pred);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 search_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Size __count,
          const _Tp& __value);
@@ -164,17 +165,17 @@ transform(_ExecutionPolicy&& __exec, _ForwardIterator1 __first1, _ForwardIterato
 
 // [alg.replace]
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 replace_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred,
            const _Tp& __new_value);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 replace(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __old_value,
         const _Tp& __new_value);
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryPredicate, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _UnaryPredicate, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator2)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 replace_copy_if(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                 _ForwardIterator2 __result, _UnaryPredicate __pred, const _Tp& __new_value);
@@ -186,11 +187,11 @@ replace_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardItera
 
 // [alg.fill]
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, void>
 fill(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Size, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 fill_n(_ExecutionPolicy&& __exec, _ForwardIterator __first, _Size __count, const _Tp& __value);
 
@@ -210,7 +211,7 @@ __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardItera
 remove_copy_if(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last,
                _ForwardIterator2 __result, _Predicate __pred);
 
-template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator1, class _ForwardIterator2, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator1)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2>
 remove_copy(_ExecutionPolicy&& __exec, _ForwardIterator1 __first, _ForwardIterator1 __last, _ForwardIterator2 __result,
             const _Tp& __value);
@@ -219,7 +220,7 @@ template <class _ExecutionPolicy, class _ForwardIterator, class _UnaryPredicate>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 remove_if(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _UnaryPredicate __pred);
 
-template <class _ExecutionPolicy, class _ForwardIterator, class _Tp>
+template <class _ExecutionPolicy, class _ForwardIterator, class _Tp _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_ForwardIterator)>
 __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator>
 remove(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
 
diff --git a/libstdc++-v3/include/std/algorithm b/libstdc++-v3/include/std/algorithm
index 163e6b5dca7..b410e7c281b 100644
--- a/libstdc++-v3/include/std/algorithm
+++ b/libstdc++-v3/include/std/algorithm
@@ -63,6 +63,7 @@
 # include <bits/ranges_algo.h>
 #endif
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_clamp
 #define __glibcxx_want_constexpr_algorithms
 #define __glibcxx_want_freestanding_algorithm
diff --git a/libstdc++-v3/include/std/deque b/libstdc++-v3/include/std/deque
index 0bf8309c19a..76d07dbf30f 100644
--- a/libstdc++-v3/include/std/deque
+++ b/libstdc++-v3/include/std/deque
@@ -68,6 +68,7 @@
 #include <bits/range_access.h>
 #include <bits/deque.tcc>
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_erase_if
 #define __glibcxx_want_nonmember_container_access
 #include <bits/version.h>
@@ -115,7 +116,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return 0;
     }
 
-  template<typename _Tp, typename _Alloc, typename _Up>
+  template<typename _Tp, typename _Alloc, typename _Up
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp)>
     inline typename deque<_Tp, _Alloc>::size_type
     erase(deque<_Tp, _Alloc>& __cont, const _Up& __value)
     {
diff --git a/libstdc++-v3/include/std/forward_list b/libstdc++-v3/include/std/forward_list
index 5ac74360808..d91a86cc797 100644
--- a/libstdc++-v3/include/std/forward_list
+++ b/libstdc++-v3/include/std/forward_list
@@ -45,6 +45,7 @@
 # include <debug/forward_list>
 #endif
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_erase_if
 #define __glibcxx_want_incomplete_container_elements
 #define __glibcxx_want_list_remove_return_type
@@ -74,7 +75,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     erase_if(forward_list<_Tp, _Alloc>& __cont, _Predicate __pred)
     { return __cont.remove_if(__pred); }
 
-  template<typename _Tp, typename _Alloc, typename _Up>
+  template<typename _Tp, typename _Alloc, typename _Up
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp)>
     inline typename forward_list<_Tp, _Alloc>::size_type
     erase(forward_list<_Tp, _Alloc>& __cont, const _Up& __value)
     {
diff --git a/libstdc++-v3/include/std/list b/libstdc++-v3/include/std/list
index fce4e3d925b..08eb453f9e0 100644
--- a/libstdc++-v3/include/std/list
+++ b/libstdc++-v3/include/std/list
@@ -69,6 +69,7 @@
 # include <debug/list>
 #endif
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_erase_if
 #define __glibcxx_want_incomplete_container_elements
 #define __glibcxx_want_list_remove_return_type
@@ -98,7 +99,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     erase_if(list<_Tp, _Alloc>& __cont, _Predicate __pred)
     { return __cont.remove_if(__pred); }
 
-  template<typename _Tp, typename _Alloc, typename _Up>
+  template<typename _Tp, typename _Alloc, typename _Up
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp)>
     inline typename list<_Tp, _Alloc>::size_type
     erase(list<_Tp, _Alloc>& __cont, const _Up& __value)
     {
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index b7c7aa36ddc..a04c830f059 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -51,6 +51,7 @@
 #include <bits/ranges_util.h>
 #include <bits/refwrap.h>
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_ranges
 #define __glibcxx_want_ranges_as_const
 #define __glibcxx_want_ranges_as_rvalue
diff --git a/libstdc++-v3/include/std/string b/libstdc++-v3/include/std/string
index 55144409cca..d8ad199b352 100644
--- a/libstdc++-v3/include/std/string
+++ b/libstdc++-v3/include/std/string
@@ -54,6 +54,7 @@
 #include <bits/basic_string.h>
 #include <bits/basic_string.tcc>
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_constexpr_char_traits
 #define __glibcxx_want_constexpr_string
 #define __glibcxx_want_erase_if
@@ -104,7 +105,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __osz - __cont.size();
     }
 
-  template<typename _CharT, typename _Traits, typename _Alloc, typename _Up>
+  template<typename _CharT, typename _Traits, typename _Alloc, typename _Up
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_CharT)>
     _GLIBCXX20_CONSTEXPR
     inline typename basic_string<_CharT, _Traits, _Alloc>::size_type
     erase(basic_string<_CharT, _Traits, _Alloc>& __cont, const _Up& __value)
diff --git a/libstdc++-v3/include/std/vector b/libstdc++-v3/include/std/vector
index a1f7ef86824..4e551408339 100644
--- a/libstdc++-v3/include/std/vector
+++ b/libstdc++-v3/include/std/vector
@@ -76,6 +76,7 @@
 # include <debug/vector>
 #endif
 
+#define __glibcxx_want_algorithm_default_value_type
 #define __glibcxx_want_constexpr_vector
 #define __glibcxx_want_erase_if
 #define __glibcxx_want_incomplete_container_elements
@@ -128,7 +129,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return 0;
     }
 
-  template<typename _Tp, typename _Alloc, typename _Up>
+  template<typename _Tp, typename _Alloc, typename _Up
+	   _GLIBCXX26_ALGORITHM_DEFAULT_VALUE_TYPE(_Tp)>
     _GLIBCXX20_CONSTEXPR
     inline typename vector<_Tp, _Alloc>::size_type
     erase(vector<_Tp, _Alloc>& __cont, const _Up& __value)
diff --git a/libstdc++-v3/testsuite/23_containers/default_template_value.cc b/libstdc++-v3/testsuite/23_containers/default_template_value.cc
new file mode 100644
index 00000000000..b7615032686
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/default_template_value.cc
@@ -0,0 +1,40 @@
+// { dg-do compile { target c++26 } }
+
+#include <deque>
+#include <forward_list>
+#include <list>
+#include <string>
+#include <vector>
+
+#if !defined(__cpp_lib_algorithm_default_value_type)
+#error "Feature test macro for default template type for algorithms' values is missing"
+#elif __cpp_lib_algorithm_default_value_type < 202403L
+#error "Feature test macro for default template type for algorithms' values is wrong"
+#endif
+
+struct S {
+  S(int, double);
+  friend auto operator<=>(const S&, const S&) = default;
+};
+
+template<template<typename...> typename Container>
+void test_erase()
+{
+  Container<S> c;
+  std::erase(c, {1, 3.14});
+}
+
+void
+test()
+{
+  test_erase<std::deque>();
+  test_erase<std::forward_list>();
+  test_erase<std::list>();
+  test_erase<std::vector>();
+
+  std::string s;
+  std::erase(s, {'x'});
+
+  std::wstring ws;
+  std::erase(ws, {L'x'});
+}
diff --git a/libstdc++-v3/testsuite/25_algorithms/default_template_value.cc b/libstdc++-v3/testsuite/25_algorithms/default_template_value.cc
new file mode 100644
index 00000000000..3cf51bc087b
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/default_template_value.cc
@@ -0,0 +1,142 @@
+// { dg-do compile { target c++26 } }
+
+#include <algorithm>
+
+#if !defined(__cpp_lib_algorithm_default_value_type)
+#error "Feature test macro for default template type for algorithms' values is missing"
+#elif __cpp_lib_algorithm_default_value_type < 202403L
+#error "Feature test macro for default template type for algorithms' values is wrong"
+#endif
+
+#include <execution>
+#include <ranges>
+#include <iterator>
+#include <vector>
+
+// Conversions from Input to Output will be used in certain algorithms.
+// Make Output have a different number of arguments to its constructor
+// so we can check whether a braced-init-list is indeed matching Input
+// or Output
+struct Output
+{
+  Output(char, float, long);
+};
+
+#define OUTPUT_VAL {'x', 1.171f, 10L}
+
+struct Input
+{
+  Input(int, double);
+  friend bool operator<=>(const Input &, const Input &) = default;
+  friend Input operator+(const Input &, const Input &);
+  operator Output() const;
+};
+
+#define INPUT_VAL {1, 3.14}
+
+void
+test()
+{
+  extern std::vector<Input> in;
+  extern std::vector<Output> out;
+
+  const auto pred = [](auto &&) { return true; };
+
+  // [alg.find]
+  (void) std::find(in.begin(), in.end(), INPUT_VAL);
+  (void) std::find(std::execution::seq, in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::find(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::find(in, INPUT_VAL);
+
+  // [alg.count]
+  (void) std::count(in.begin(), in.end(), INPUT_VAL);
+  (void) std::count(std::execution::seq, in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::count(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::count(in, INPUT_VAL);
+
+  // [alg.search]
+  (void) std::search_n(in.begin(), in.end(), 10, INPUT_VAL);
+  (void) std::search_n(in.begin(), in.end(), 10, INPUT_VAL, std::equal_to{});
+  (void) std::search_n(std::execution::seq, in.begin(), in.end(), 10, INPUT_VAL);
+  (void) std::search_n(std::execution::seq, in.begin(), in.end(), 10, INPUT_VAL, std::equal_to{});
+  (void) std::ranges::search_n(in.begin(), in.end(), 10, INPUT_VAL);
+  (void) std::ranges::search_n(in, 10, INPUT_VAL);
+
+  // [alg.replace]
+  (void) std::replace(in.begin(), in.end(), INPUT_VAL, INPUT_VAL);
+  (void) std::replace(std::execution::seq, in.begin(), in.end(), INPUT_VAL, INPUT_VAL);
+  (void) std::replace_if(in.begin(), in.end(), pred, INPUT_VAL);
+  (void) std::replace_if(std::execution::seq, in.begin(), in.end(), pred, INPUT_VAL);
+
+  (void) std::ranges::replace(in.begin(), in.end(), INPUT_VAL, INPUT_VAL);
+  (void) std::ranges::replace(in, INPUT_VAL, INPUT_VAL);
+  (void) std::ranges::replace_if(in.begin(), in.end(), pred, INPUT_VAL);
+  (void) std::ranges::replace_if(in, pred, INPUT_VAL);
+
+  (void) std::replace_copy_if(in.begin(), in.end(), out.begin(), pred, OUTPUT_VAL);
+  (void) std::replace_copy_if(std::execution::seq, in.begin(), in.end(), out.begin(), pred, OUTPUT_VAL);
+  (void) std::ranges::replace_copy_if(in.begin(), in.end(), out.begin(), pred, OUTPUT_VAL);
+  (void) std::ranges::replace_copy_if(in, out.begin(), pred, OUTPUT_VAL);
+
+  // Non-range replace_copy is deliberately skipped by P2248
+  (void) std::ranges::replace_copy(in.begin(), in.end(), out.begin(), INPUT_VAL, OUTPUT_VAL);
+  (void) std::ranges::replace_copy(in, out.begin(), INPUT_VAL, OUTPUT_VAL);
+
+  // [alg.fill]
+  (void) std::fill(in.begin(), in.end(), INPUT_VAL);
+  (void) std::fill(std::execution::seq, in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::fill(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::fill(in, INPUT_VAL);
+
+  (void) std::fill_n(in.begin(), 10, INPUT_VAL);
+  (void) std::fill_n(std::execution::seq, in.begin(), 10, INPUT_VAL);
+  (void) std::ranges::fill_n(in.begin(), 10, INPUT_VAL);
+
+  // [alg.remove]
+  (void) std::remove(in.begin(), in.end(), INPUT_VAL);
+  (void) std::remove(std::execution::seq, in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::remove(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::remove(in, INPUT_VAL);
+
+  (void) std::remove_copy(in.begin(), in.end(), out.begin(), INPUT_VAL);
+  (void) std::remove_copy(std::execution::seq, in.begin(), in.end(), out.begin(), INPUT_VAL);
+  (void) std::ranges::remove_copy(in.begin(), in.end(), out.begin(), INPUT_VAL);
+  (void) std::ranges::remove_copy(in, out.begin(), INPUT_VAL);
+
+  // [alg.binary.search]
+  (void) std::lower_bound(in.begin(), in.end(), INPUT_VAL);
+  (void) std::lower_bound(in.begin(), in.end(), INPUT_VAL, std::less{});
+  (void) std::ranges::lower_bound(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::lower_bound(in, INPUT_VAL);
+
+  (void) std::upper_bound(in.begin(), in.end(), INPUT_VAL);
+  (void) std::upper_bound(in.begin(), in.end(), INPUT_VAL, std::less{});
+  (void) std::ranges::upper_bound(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::upper_bound(in, INPUT_VAL);
+
+  (void) std::equal_range(in.begin(), in.end(), INPUT_VAL);
+  (void) std::equal_range(in.begin(), in.end(), INPUT_VAL, std::less{});
+  (void) std::ranges::equal_range(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::equal_range(in, INPUT_VAL);
+
+  (void) std::binary_search(in.begin(), in.end(), INPUT_VAL);
+  (void) std::binary_search(in.begin(), in.end(), INPUT_VAL, std::less{});
+  (void) std::ranges::binary_search(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::binary_search(in, INPUT_VAL);
+
+  // [alg.fold]
+  (void) std::ranges::fold_left(in.begin(), in.end(), INPUT_VAL, std::plus<>{});
+  (void) std::ranges::fold_left(in, INPUT_VAL, std::plus<>{});
+  (void) std::ranges::fold_right(in.begin(), in.end(), INPUT_VAL, std::plus<>{});
+  (void) std::ranges::fold_right(in, INPUT_VAL, std::plus<>{});
+  (void) std::ranges::fold_left_with_iter(in.begin(), in.end(), INPUT_VAL, std::plus<>{});
+  (void) std::ranges::fold_left_with_iter(in, INPUT_VAL, std::plus<>{});
+
+  // [alg.contains]
+  (void) std::ranges::contains(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::contains(in, INPUT_VAL);
+
+  // [alg.find.last]
+  (void) std::ranges::find_last(in.begin(), in.end(), INPUT_VAL);
+  (void) std::ranges::find_last(in, INPUT_VAL);
+}
-- 
2.34.1

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