[Bug libstdc++/95200] New: user-defined hash function is not copied correctly if unordered_map is declared using an incomplete type

[jevgenijsz1 at verifone dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95200

            Bug ID: 95200
           Summary: user-defined hash function is not copied correctly if
                    unordered_map is declared using an incomplete type
           Product: gcc
           Version: 9.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jevgenijsz1 at verifone dot com
  Target Milestone: ---

Created attachment 48558
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48558&action=edit
g++ -v output

The failing code requires at least 3 source files, I did not find a cleaner way
to reproduce the issue. I'm not 100% sure this is a bug in libstdc++ however I
couldn't find a reason for why this should not work as expected.

Code:
=====
map_obj.h
=====

#ifndef MAP_OBJ_H_
#define MAP_OBJ_H_

#include <unordered_map>

enum class EnumType;

typedef std::unordered_map<EnumType, bool> map;

class MapObj {
 public:
  map getEnumMap() const;
  map enum_map_;
};

#endif
=====

=====
map_obj.cpp
=====
#include "map_obj.h"

map MapObj::getEnumMap() const {
  return enum_map_;
}
=====

=====
main.cpp
=====
#include <functional>

enum class EnumType : int {
  VALUE0,
  VALUE1,
};

namespace std {
template <>
struct hash<EnumType> {
  size_t operator()(EnumType type) const { return
std::hash<int>()(static_cast<int>(type)); }
};
}

#include "map_obj.h"

int main() {
  auto event = MapObj();
  event.enum_map_[EnumType::VALUE0] = true;
  event.enum_map_[EnumType::VALUE1] = false;
  event.getEnumMap().at(EnumType::VALUE0);
  event.getEnumMap().at(EnumType::VALUE1);
  return 0;
}
=====

using: 
gcc version 9.3.1 20200408 (Red Hat 9.3.1-2) (GCC)
libstdc++ 3.4.28

command line to build: g++ -save-temps -Wall -Wextra -o map_test main.cpp
map_obj.cpp

expected result: runnnig map_test returns 0

actual result: terminate called after throwing an instance of
'std::out_of_range'
  what():  _Map_base::at