[gcc r14-6200] libstdc++: Add workaround to std::ranges::subrange [PR111948]

[Jonathan Wakely]

https://gcc.gnu.org/g:08448dc146b6dd32383d64ab491a594d41f62aaa

commit r14-6200-g08448dc146b6dd32383d64ab491a594d41f62aaa
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Oct 24 20:15:12 2023 +0100

    libstdc++: Add workaround to std::ranges::subrange [PR111948]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/111948
            * include/bits/ranges_util.h (subrange): Add constructor to
            _Size to aoid setting member in constructor.
            * testsuite/std/ranges/subrange/111948.cc: New test.

Diff:
---
 libstdc++-v3/include/bits/ranges_util.h             | 21 +++++++++++++--------
 .../testsuite/std/ranges/subrange/111948.cc         |  8 ++++++++
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index ab6c69c57d0..185e46ec7a9 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -267,13 +267,21 @@ namespace ranges
       using __size_type
 	= __detail::__make_unsigned_like_t<iter_difference_t<_It>>;
 
-      template<typename, bool = _S_store_size>
+      template<typename _Tp, bool = _S_store_size>
 	struct _Size
-	{ };
+	{
+	  [[__gnu__::__always_inline__]]
+	  constexpr _Size(_Tp = {}) { }
+	};
 
       template<typename _Tp>
 	struct _Size<_Tp, true>
-	{ _Tp _M_size; };
+	{
+	  [[__gnu__::__always_inline__]]
+	  constexpr _Size(_Tp __s = {}) : _M_size(__s) { }
+
+	  _Tp _M_size;
+	};
 
       [[no_unique_address]] _Size<__size_type> _M_size = {};
 
@@ -294,11 +302,8 @@ namespace ranges
       noexcept(is_nothrow_constructible_v<_It, decltype(__i)>
 	       && is_nothrow_constructible_v<_Sent, _Sent&>)
 	requires (_Kind == subrange_kind::sized)
-      : _M_begin(std::move(__i)), _M_end(__s)
-      {
-	if constexpr (_S_store_size)
-	  _M_size._M_size = __n;
-      }
+      : _M_begin(std::move(__i)), _M_end(__s), _M_size(__n)
+      { }
 
       template<__detail::__different_from<subrange> _Rng>
 	requires borrowed_range<_Rng>
diff --git a/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc b/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc
new file mode 100644
index 00000000000..dcc64b56def
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/subrange/111948.cc
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++20 } }
+
+#include <ranges>
+
+// Bug libstdc++/111948 - subrange modifies a const size object
+
+constexpr auto r = std::ranges::subrange(std::views::iota(0), 5);
+static_assert(std::ranges::distance(r));