[Bug target/113960] std::map with std::vector as input overwrites itself with c++20, on s390x platform

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113960

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1824,11 +1824,14 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
     }

 #if __cpp_lib_three_way_comparison
-  // Iter points to a contiguous range of unsigned narrow character type
-  // or std::byte, suitable for comparison by memcmp.
-  template<typename _Iter>
-    concept __is_byte_iter = contiguous_iterator<_Iter>
-      && __is_memcmp_ordered<iter_value_t<_Iter>>::__value;
+  // Both iterators refer to contiguous ranges of unsigned narrow characters,
+  // or std::byte, or big-endian unsigned integers, suitable for comparison
+  // using memcmp.
+  template<typename _Iter1, typename _Iter2>
+    concept __memcmp_ordered_with
+      = (__is_memcmp_ordered_with<iter_value_t<_Iter1>,
+                                 iter_value_t<_Iter2>>::__value)
+         && contiguous_iterator<_Iter1> && contiguous_iterator<_Iter2>;

   // Return a struct with two members, initialized to the smaller of x and y
   // (or x if they compare equal) and the result of the comparison x <=> y.
@@ -1878,20 +1881,20 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       if (!std::__is_constant_evaluated())
        if constexpr (same_as<_Comp, __detail::_Synth3way>
                      || same_as<_Comp, compare_three_way>)
-         if constexpr (__is_byte_iter<_InputIter1>)
-           if constexpr (__is_byte_iter<_InputIter2>)
-             {
-               const auto [__len, __lencmp] = _GLIBCXX_STD_A::
-                 __min_cmp(__last1 - __first1, __last2 - __first2);
-               if (__len)
-                 {
-                   const auto __c
-                     = __builtin_memcmp(&*__first1, &*__first2, __len) <=> 0;
-                   if (__c != 0)
-                     return __c;
-                 }
-               return __lencmp;
-             }
+         if constexpr (__memcmp_ordered_with<_InputIter1, _InputIter2>)
+           {
+             const auto [__len, __lencmp] = _GLIBCXX_STD_A::
+               __min_cmp(__last1 - __first1, __last2 - __first2);
+             if (__len)
+               {
+                 const auto __blen = __len * sizeof(*__first1);
+                 const auto __c
+                   = __builtin_memcmp(&*__first1, &*__first2, __blen) <=> 0;
+                 if (__c != 0)
+                   return __c;
+               }
+             return __lencmp;
+           }

       while (__first1 != __last1)
        {