在SFINAE中将int缩小为bool，gcc和clang之间的输出不同

问题描述

请考虑以下示例:

template<int i>
struct nice_type;

template<class T>
struct is_nice : std::false_type {};

template<int i>
struct is_nice< nice_type<i> > : std::integral_constant<int, i> {};

template<class T, class = void>
struct pick
{
    typedef std::integral_constant<int, -1> type;
};

template<class T>
struct pick<T, typename std::enable_if< is_nice<T>::value >::type >
{
    typedef std::integral_constant<int, is_nice<T>::value > type;
};

int main()
{
    std::cout << pick<int>::type::value << ", ";
    std::cout << pick< nice_type<42> >::type::value << std::endl;
    return 0;
}

Clang(3.4.1)输出"-1，-1"，而GCC(4.9.0)输出"-1，42".

Clang (3.4.1) outputs "-1, -1", while GCC(4.9.0) outputs "-1, 42".

问题在于 pick 的专业化.虽然Gcc似乎很乐意将 is_nice< T> :: value (42)转换为 bool(true)，但是clang却不这样做，并放弃了专业化.这两个示例均使用 -std = c ++ 11 编译.

The problem lays in the specialization of pick. While Gcc seems happy to convert is_nice<T>::value (42) to bool(true), clang does not do so, and discards the specialization. Both examples compiled with -std=c++11.

哪个编译器是正确的?

推荐答案

这是gcc错误 57891 .将整数常量 42 转换为 bool 涉及缩小转换，这是非类型模板参数所不允许的.因此， enable_if 的格式不正确，应该丢弃 pick 的专业化名称，就像clang正确进行的那样.

This is gcc bug 57891. Conversion of the integral constant 42 to bool involves a narrowing conversion, which is not allowed in non-type template arguments. Hence the enable_if is ill-formed, and the pick specialization should be discarded, as clang correctly does.

§14.3.2/5 [temp.arg.nontype]

§5.19/3 [expr.const]

§8.5.4/7 [dcl.init.list]

---

此最小示例演示了gcc错误:

template<bool>
struct foo{};
foo<10> f;

int main() {}

gcc-4.9接受代码，而clang-3.4拒绝它并出现以下错误:

gcc-4.9 accepts the code while clang-3.4 rejects it with the following error:

 foo<10> f;
     ^

---

轻松解决您的特定问题.确保 enable_if 的非类型模板参数的计算结果为 bool

---

The fix to your particular problem is easy. Make sure the non-type template argument to enable_if evaluates to a bool

template<class T>
struct pick<T, typename std::enable_if< is_nice<T>::value != 0 >::type >
//                                                       ^^^^^^
{
    typedef std::integral_constant<int, is_nice<T>::value > type;
};

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