gcc 4.7 STL库对执行缺陷？

Question

以下代码在gcc 4.6上编译但不在4.7上编译.这是4.7的问题还是4.6的问题？用-std = gnu ++ 0x编译.

#include <utility>

using namespace std;

struct Z {
};

struct X {
    operator Z*() const { return nullptr; }
};

struct Y {
    Y(Z*) {}
};

int main() {
    pair<int, Y> p(make_pair(0, X()));
}

错误消息:

[hidden]$ g++-mp-4.6 -std=gnu++0x e.cpp 
[hidden]$ g++-mp-4.7 -std=gnu++0x e.cpp 
e.cpp: In function 'int main()':
e.cpp:17:37: error: no matching function for call to 'std::pair<int, Y>::pair(std::pair<int, X>)'
e.cpp:17:37: note: candidates are:
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:204:9: note: template<class ... _Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int ..._Indexes2> std::pair::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>)
/opt/local/include/gcc47/c++/bits/stl_pair.h:204:9: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   'std::pair<int, X>' is not derived from 'std::tuple<_Args1 ...>'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:153:9: note: template<class ... _Args1, class ... _Args2> std::pair::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>)
/opt/local/include/gcc47/c++/bits/stl_pair.h:153:9: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   cannot convert 'std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = X; typename std::__decay_and_strip<_T2>::__type = X; typename std::__decay_and_strip<_T1>::__type = int]((* & X()))' (type 'std::pair<int, X>') to type 'std::piecewise_construct_t'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:148:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(std::pair<_U1, _U2>&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:148:12: note:   template argument deduction/substitution failed:
/opt/local/include/gcc47/c++/bits/stl_pair.h:145:38: error: no type named 'type' in 'struct std::enable_if<false, void>'
/opt/local/include/gcc47/c++/bits/stl_pair.h:142:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(_U1&&, _U2&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:142:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   candidate expects 2 arguments, 1 provided
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:136:12: note: template<class _U2, class> constexpr std::pair::pair(const _T1&, _U2&&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:136:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   cannot convert 'std::make_pair(_T1&&, _T2&&) [with _T1 = int; _T2 = X; typename std::__decay_and_strip<_T2>::__type = X; typename std::__decay_and_strip<_T1>::__type = int]((* & X()))' (type 'std::pair<int, X>') to type 'const int&'
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:131:12: note: template<class _U1, class> constexpr std::pair::pair(_U1&&, const _T2&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:131:12: note:   template argument deduction/substitution failed:
e.cpp:17:37: note:   candidate expects 2 arguments, 1 provided
In file included from /opt/local/include/gcc47/c++/utility:72:0,
                 from e.cpp:1:
/opt/local/include/gcc47/c++/bits/stl_pair.h:122:7: note: std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = int; _T2 = Y; std::pair<_T1, _T2> = std::pair<int, Y>]
/opt/local/include/gcc47/c++/bits/stl_pair.h:122:7: note:   no known conversion for argument 1 from 'std::pair<int, X>' to 'std::pair<int, Y>&&'
/opt/local/include/gcc47/c++/bits/stl_pair.h:119:17: note: constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = Y; std::pair<_T1, _T2> = std::pair<int, Y>]
/opt/local/include/gcc47/c++/bits/stl_pair.h:119:17: note:   no known conversion for argument 1 from 'std::pair<int, X>' to 'const std::pair<int, Y>&'
/opt/local/include/gcc47/c++/bits/stl_pair.h:116:12: note: template<class _U1, class _U2, class> constexpr std::pair::pair(const std::pair<_U1, _U2>&)
/opt/local/include/gcc47/c++/bits/stl_pair.h:116:12: note:   template argument deduction/substitution failed:
/opt/local/include/gcc47/c++/bits/stl_pair.h:113:38: error: no type named 'type' in 'struct std::enable_if<false, void>'
/opt/local/include/gcc47/c++/bits/stl_pair.h:104:26: note: constexpr std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int; _T2 = Y]
/opt/local/include/gcc47/c++/bits/stl_pair.h:104:26: note:   candidate expects 2 arguments, 1 provided
/opt/local/include/gcc47/c++/bits/stl_pair.h:100:26: note: constexpr std::pair<_T1, _T2>::pair() [with _T1 = int; _T2 = Y]
/opt/local/include/gcc47/c++/bits/stl_pair.h:100:26: note:   candidate expects 0 arguments, 1 provided

Answer 1

那不应该编译.

初始化p.second需要隐式转换X为Y.隐式转换最多只能涉及一个用户定义的转换所需的转换需要两个转换; X要Z*通过转换操作符,并Z*以Y通过转换构造函数.

只允许通过隐式转换来初始化来自另一对的对元素.C++ 11说:

20.3.2/12,此构造不得参与重载解析,除非const U&是隐式转换为first_type和const V&是隐式转换到second_type.

和C++ 98说:

20.2.2/4从参数的相应成员初始化成员,根据需要执行隐式转换.

据推测,旧版本有一个错误,允许考虑转换,并且该错误已在更新版本中修复.