[libcxx] calling find() on a map with a non-empty transparent comparator causes a compilation failure (found in PoDoFo)

[vimproved]

In libc++ 21.1.0_rc2, calling find() on a map with a non-empty transparent comparator causes a compilation failure (no member named '__get_value' in 'std::pair<const std::string, int>'). The code below compiles just fine with libc++ 20.1.8, but fails with 21.1.0_rc2. This bug was found in the wild in PoDoFo 0.10.3, see https://bugs.gentoo.org/961184.

#include <map>
#include <string>
#include <string_view>

struct my_comp : std::less<> {
  int i;
};

auto test(std::map<std::string, int, my_comp>& m, std::string_view v) {
  m.find(v);
}

compilation error

In file included from test.cpp:1:
/usr/include/c++/v1/map:700:24: error: no member named '__get_value' in 'std::pair<const std::string, int>'
  700 |     return __comp_(__x.__get_value().first, __y);
      |                    ~~~ ^
/usr/include/c++/v1/__tree:2050:10: note: in instantiation of function template specialization 'std::__map_value_compare<std::string, std::pair<const std::string,
      int>, my_comp>::operator()<std::string_view>' requested here
 2050 |     if (!value_comp()(__root->__value_, __v)) {
      |          ^
/usr/include/c++/v1/__tree:1993:18: note: in instantiation of function template specialization 'std::__tree<std::__value_type<std::string, int>,
      std::__map_value_compare<std::string, std::pair<const std::string, int>, my_comp>, std::allocator<std::pair<const std::string, int>>>::__lower_bound<std::string_view>' requested here
 1993 |   iterator __p = __lower_bound(__v, __root(), __end_node());
      |                  ^
/usr/include/c++/v1/map:1249:20: note: in instantiation of function template specialization 'std::__tree<std::__value_type<std::string, int>,
      std::__map_value_compare<std::string, std::pair<const std::string, int>, my_comp>, std::allocator<std::pair<const std::string, int>>>::find<std::string_view>' requested here
 1249 |     return __tree_.find(__k);
      |                    ^
test.cpp:8:5: note: in instantiation of function template specialization 'std::map<std::string, int, my_comp>::find<std::string_view, 0>' requested here
    8 |   m.find(v);
      |     ^
In file included from test.cpp:1:
/usr/include/c++/v1/map:695:29: error: no member named '__get_value' in 'std::pair<const std::string, int>'
  695 |     return __comp_(__x, __y.__get_value().first);
      |                         ~~~ ^
/usr/include/c++/v1/__tree:1994:24: note: in instantiation of function template specialization 'std::__map_value_compare<std::string, std::pair<const std::string,
      int>, my_comp>::operator()<std::string_view>' requested here
 1994 |   if (__p != end() && !value_comp()(__v, *__p))
      |                        ^
/usr/include/c++/v1/map:1249:20: note: in instantiation of function template specialization 'std::__tree<std::__value_type<std::string, int>,
      std::__map_value_compare<std::string, std::pair<const std::string, int>, my_comp>, std::allocator<std::pair<const std::string, int>>>::find<std::string_view>' requested here
 1249 |     return __tree_.find(__k);
      |                    ^
test.cpp:8:5: note: in instantiation of function template specialization 'std::map<std::string, int, my_comp>::find<std::string_view, 0>' requested here
    8 |   m.find(v);
      |     ^
2 errors generated.

[frederick-vs-ja]

Looks like a missed change in #134819.