неоднозначная перегрузка для «оператора =» с помощью c ++ 11 std :: move и копирования и замены идиомы

Я получаю следующую ошибку:

[matt ~] g++ -std=c++11 main.cpp -DCOPY_AND_SWAP && ./a.out
main.cpp: In function ‘int main(int, const char* const*)’:
main.cpp:101:24: error: ambiguous overload for ‘operator=’ in ‘move = std::move<Test&>((* & copy))’
main.cpp:101:24: note: candidates are:
main.cpp:39:7: note: Test& Test::operator=(Test)
main.cpp:52:7: note: Test& Test::operator=(Test&&)

Когда следующий код скомпилирован:

#include <iostream>
#include <unordered_map>
class Test final {
public:
typedef std::unordered_map<std::string, std::string> Map;
public:
Test();
explicit Test(Map&& map);
~Test();
Test(const Test& other);
Test(Test&& test);
#ifdef COPY_AND_SWAP
Test& operator=(Test other);
#else
Test& operator=(const Test& other);
#endif
Test& operator=(Test&& other);
size_t Size() const noexcept;
friend void swap(Test& lhs, Test& rhs);
private:
friend std::ostream& operator<<(std::ostream& stream, const Test& test);
private:
Map map_;
};
Test::Test() : map_() {
std::cerr << "Default constructor called" << std::endl;
};
Test::Test(const Test& other) : map_(other.map_) {
std::cerr << "Copy constructor called" << std::endl;
};
Test::Test(Test&& other) : map_(std::move(other.map_)) {
std::cerr << "Move constructor called" << std::endl;
};
Test::Test(Map&& map) : map_(std::move(map)) {
std::cerr << "Map constructor called" << std::endl;
};
Test::~Test() {};
#ifdef COPY_AND_SWAP
Test& Test::operator=(Test other) {
std::cerr << "Copy and swap assignment called" << std::endl;
using std::swap;
swap(this->map_, other.map_);
return *this;
}
#else
Test& Test::operator=(const Test& other) {
std::cerr << "Copy assignment called" << std::endl;
this->map_ = other.map_;
return *this;
}
#endif
Test& Test::operator=(Test&& other) {
std::cerr << "Move assignment called" << std::endl;
this->map_ = other.map_;
other.map_.clear();
return *this;
}
size_t Test::Size() const noexcept {
return map_.size();
}
void swap(Test& lhs, Test& rhs) {
using std::swap;
swap(lhs.map_, rhs.map_);
}
std::ostream& operator<<(std::ostream& stream, const Test& test) {
return stream << test.map_.size();
}
int main (const int argc, const char * const * const argv) {
using std::swap;
Test::Map map {
{"some", "dummy"},
{"data", "to"},
{"fill", "up"},
{"the", "map"}
};
std::cout << " map size(): " << map.size() << std::endl;
std::cout << "Constructing" << std::endl;
Test test(std::move(map));
std::cout << " map.size(): " << map.size() << std::endl;
std::cout << "test.Size(): " << test.Size() << std::endl;
std::cout << "Copy construction" << std::endl;
Test copy(test);
std::cout << "copy.Size(): " << copy.Size() << std::endl;
std::cout << "Move construction" << std::endl;
Test move(std::move(copy));
std::cout << "move.Size(): " << move.Size() << std::endl;
std::cout << "copy.Size(): " << copy.Size() << std::endl;
std::cout << "Swapping" << std::endl;
swap(move, copy);
std::cout << "move.Size(): " << move.Size() << std::endl;
std::cout << "copy.Size(): " << copy.Size() << std::endl;
std::cout << "Swapping back" << std::endl;
swap(move, copy);
std::cout << "move.Size(): " << move.Size() << std::endl;
std::cout << "copy.Size(): " << copy.Size() << std::endl;
std::cout << "Copy assignment" << std::endl;
copy = test;
std::cout << "test.Size(): " << test.Size() << std::endl;
std::cout << "copy.Size(): " << copy.Size() << std::endl;
std::cout << "Move assignment" << std::endl;
move = std::move(copy);
std::cout << "move.Size(): " << move.Size() << std::endl;
std::cout << "copy.Size(): " << copy.Size() << std::endl;
return 0;
}

Когда скомпилировано с g++ -std=c++11 main.cpp && ./a.out:

[matt ~] g++ -std=c++11 main.cpp && ./a.out
map size(): 4
Constructing
Map constructor called
map.size(): 0
test.Size(): 4
Copy construction
Copy constructor called
copy.Size(): 4
Move construction
Move constructor called
move.Size(): 4
copy.Size(): 0
Swapping
move.Size(): 0
copy.Size(): 4
Swapping back
move.Size(): 4
copy.Size(): 0
Copy assignment
Copy assignment called
test.Size(): 4
copy.Size(): 4
Move assignment
Move assignment called
move.Size(): 4
copy.Size(): 0

Может ли кто-нибудь помочь мне понять, почему возникает двусмысленность при использовании идиомы копирования и замены в этом случае?