为什么 `static_assert` 总是被调用？

Question

如果 USE_STATIC_ASSERT 是 0，这会按预期工作（从列表中获取索引类型）。如果 1 则 static_assert() 总是跳闸。我原以为 static_assert() 只有在所有 typename 都用完时才会发生。为什么不是这样？

#define USE_STATIC_ASSERT 1
template <unsigned int I, typename ...Ts>
struct items;

template <typename T, typename ...Ts>
struct items<0, T, Ts...>
{
    typedef T type;
};

template <unsigned int I, typename T, typename ...Ts>
struct items<I, T, Ts...> : items<I-1, Ts...>
{
};

#if USE_STATIC_ASSERT
template <unsigned int I>
struct items<I>
{
    static_assert(false, "Ran out of Ts.");
};
#endif


int main()
{
    cout << is_same<float, items<1, int, float, double>::type>::value << endl;
}

Answer 1

即使未实例化包含 static_assert 的 items 的部分特化，编译器也可以根据 §14.6 [temp.res]/p8 拒绝此代码：

Knowing which names are type names allows the syntax of every template to be checked. No diagnostic shall be issued for a template for which a valid specialization can be generated. If no valid specialization can be generated for a template, and that template is not instantiated, the template is ill-formed, no diagnostic required.

要解决这个问题，您可以使 static_assert 中的表达式依赖于其他 class 模板：

#include <type_traits>

template <unsigned int I>
struct AlwaysFalse : std::false_type {};

template <unsigned int I>
struct items<I>
{
    static_assert(AlwaysFalse<I>{}, "Ran out of Ts.");
    //            ~~~~~~~~~~~~~~~^
};

为什么 `static_assert` 总是被调用？

Why does the `static_assert` always get invoked?

c++

static-assert

c++11