[gcc r13-4527] libstdc++: Add casts for integer-like difference type [PR107871]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:9cce91a63dcc40cb39e68f6a771e891e51c51946

commit r13-4527-g9cce91a63dcc40cb39e68f6a771e891e51c51946
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 5 21:38:53 2022 +0000

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

Diff:
---
 libstdc++-v3/include/std/format                       |  8 ++++----
 libstdc++-v3/testsuite/std/format/functions/107871.cc | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 4 deletions(-)

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