[gcc r13-9484] libstdc++: Add ranges::range_common_reference_t for C++20 (LWG 3860)

[Jonathan Wakely]

https://gcc.gnu.org/g:bcfc60d64d6362e5c31a8a9f11296ab65c5ae3ab

commit r13-9484-gbcfc60d64d6362e5c31a8a9f11296ab65c5ae3ab
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 12 15:02:14 2024 +0100

    libstdc++: Add ranges::range_common_reference_t for C++20 (LWG 3860)
    
    LWG 3860 added this alias template. Both libc++ and MSVC treat this as a
    DR for C++20, so this change does so too.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_base.h (range_common_reference_t): New
            alias template, as per LWG 3860.
            * testsuite/std/ranges/range.cc: Check it.
    
    (cherry picked from commit 92b554a8412624a0aa3ca9b502976ebec7eff34e)

Diff:
---
 libstdc++-v3/include/bits/ranges_base.h    | 6 ++++++
 libstdc++-v3/testsuite/std/ranges/range.cc | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 698dac7b7c1f..14237d2a3162 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -539,6 +539,12 @@ namespace ranges
     using range_rvalue_reference_t
       = iter_rvalue_reference_t<iterator_t<_Range>>;
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3860. range_common_reference_t is missing
+  template<range _Range>
+    using range_common_reference_t
+      = iter_common_reference_t<iterator_t<_Range>>;
+
   /// [range.sized] The sized_range concept.
   template<typename _Tp>
     concept sized_range = range<_Tp>
diff --git a/libstdc++-v3/testsuite/std/ranges/range.cc b/libstdc++-v3/testsuite/std/ranges/range.cc
index f721ec38718f..76a8b8d38499 100644
--- a/libstdc++-v3/testsuite/std/ranges/range.cc
+++ b/libstdc++-v3/testsuite/std/ranges/range.cc
@@ -84,3 +84,9 @@ static_assert( same_as<std::ranges::range_rvalue_reference_t<I>,
 		       char&&> );
 static_assert( same_as<std::ranges::range_rvalue_reference_t<O>,
 		      WritableObject<char>> );
+
+// LWG 3860. range_common_reference_t is missing
+static_assert( same_as<std::ranges::range_common_reference_t<C>,
+		       char&> );
+static_assert( same_as<std::ranges::range_common_reference_t<I>,
+		       char&> );