[committed] libstdc++: Add casts for integer-like difference type [PR107871]

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux. Pushed to trunk.

-- >8 --

libstdc++-v3/ChangeLog:

        PR libstdc++/107871
        * include/std/format (_Iter_sink::_M_overflow): Add cast to
        size_t.
        (_Iter_sink<CharT, contiguous_iterator auto>::_M_make_span): Use
        typedef instead of decltype.
        * testsuite/std/format/functions/107871.cc: New test.
---
 libstdc++-v3/include/std/format                    |  8 ++++----
 .../testsuite/std/format/functions/107871.cc       | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/format/functions/107871.cc

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 6d6a770eb8c..1072e2c17db 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2481,12 +2481,12 @@ namespace __format
        auto __s = this->_M_used();
        if (_M_max < 0) // No maximum.
          _M_out = ranges::copy(__s, std::move(_M_out)).out;
-       else if (_M_count < size_t(_M_max))
+       else if (_M_count < static_cast<size_t>(_M_max))
          {
            auto __max = _M_max - _M_count;
            span<_CharT> __first;
            if (__max < __s.size())
-             __first = __s.first(__max);
+             __first = __s.first(static_cast<size_t>(__max));
            else
              __first = __s;
            _M_out = ranges::copy(__first, std::move(_M_out)).out;
@@ -2564,11 +2564,11 @@ namespace __format
 
        if (__n > 0)
          {
-           if constexpr (!is_integral_v<decltype(__n)>
+           if constexpr (!is_integral_v<iter_difference_t<_OutIter>>
                            || sizeof(__n) > sizeof(size_t))
              {
                // __int128 or __detail::__max_diff_type
-               auto __m = (decltype(__n))(size_t)-1;
+               auto __m = iter_difference_t<_OutIter>((size_t)-1);
                if (__n > __m)
                  __n = __m;
              }
diff --git a/libstdc++-v3/testsuite/std/format/functions/107871.cc 
b/libstdc++-v3/testsuite/std/format/functions/107871.cc
new file mode 100644
index 00000000000..1fb558e7ac6
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/format/functions/107871.cc
@@ -0,0 +1,14 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <format>
+
+struct O {
+  using difference_type = std::ranges::__detail::__max_diff_type;
+  O& operator=(const char&);
+  O& operator*();
+  O& operator++();
+  O& operator++(int);
+};
+
+auto str = std::format_to_n(O{}, 4, "{}", " "); // PR libstdc++/107871
-- 
2.38.1