[committed] libstdc++: Make COW string use allocator_traits for nested types

[Jonathan Wakely via Gcc-patches]

When compiled as C++20 the COW std::string fails due to assuming that
the allocator always defines size_type and difference_type. That has
been incorrect since C++11, but we got away with it for specializations
using std::allocator until those members were removed in C++20.

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.h (size_type, difference_type):
        Use allocator_traits to obtain the allocator's size_type and
        difference_type.

Tested powerpc64le-linux. Committed to trunk.



commit 684d6ee140af6585c18c8790f8f5bddfcc6bd153
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu Jul 30 20:58:09 2020

    libstdc++: Make COW string use allocator_traits for nested types
    
    When compiled as C++20 the COW std::string fails due to assuming that
    the allocator always defines size_type and difference_type. That has
    been incorrect since C++11, but we got away with it for specializations
    using std::allocator until those members were removed in C++20.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (size_type, difference_type):
            Use allocator_traits to obtain the allocator's size_type and
            difference_type.

diff --git a/libstdc++-v3/include/bits/basic_string.h 
b/libstdc++-v3/include/bits/basic_string.h
index bc0c256b65e..52cecdd7ca1 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3147,8 +3147,8 @@ _GLIBCXX_END_NAMESPACE_CXX11
       typedef _Traits                                      traits_type;
       typedef typename _Traits::char_type                  value_type;
       typedef _Alloc                                       allocator_type;
-      typedef typename _CharT_alloc_type::size_type        size_type;
-      typedef typename _CharT_alloc_type::difference_type   difference_type;
+      typedef typename _CharT_alloc_traits::size_type      size_type;
+      typedef typename _CharT_alloc_traits::difference_type difference_type;
 #if __cplusplus < 201103L
       typedef typename _CharT_alloc_type::reference        reference;
       typedef typename _CharT_alloc_type::const_reference   const_reference;