[PATCH] libstdc++: Add workaround to std::ranges::subrange [PR111948]

[Jonathan Wakely]

I think I'll push this to work around the compiler bug. We can revert it
later if the front end gets fixed.

Tested x86_64-linux. Needed on trunk and gcc-13.

-- >8 --

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.
---
 libstdc++-v3/include/bits/ranges_util.h       | 21 ++++++++++++-------
 .../testsuite/std/ranges/subrange/111948.cc   |  8 +++++++
 2 files changed, 21 insertions(+), 8 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/ranges/subrange/111948.cc

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));
-- 
2.43.0