c++ - 为什么我可以覆盖 -(否定)和! (不是)但不是~(按位不是)？

Question

为什么下面定义了!和-操作符，但是没有定义~操作符？

#include <type_traits>
#include <cstdint>
#include <typeinfo>
#include <cstdio>

template <typename T, T v>
struct integral_constant
    : std::integral_constant<T, v>
{
};

#define DECL_UNARY_OP(op) \
    template <typename T, T t> \
    constexpr integral_constant<decltype(op t), (op t)> \
    operator op(integral_constant<T, t>) \
    { return {}; } \


DECL_UNARY_OP(~);
DECL_UNARY_OP(-);
DECL_UNARY_OP(!);

int main() {
    constexpr auto x = integral_constant<uint8_t, 1>{};
    constexpr auto y = integral_constant<uint8_t, 10>{};
    constexpr auto z = integral_constant<uint8_t, 100>{};
    puts(typeid(-x).name());  // integral_constant<...>
    puts(typeid(~y).name());  // int!
    puts(typeid(!z).name());  // integral_constant<...>
}

在 GCC-4.8.2 上编译它会得到 the following on godbolt ，您可以清楚地看到中间操作已经从 integral_constant 类型衰减。

为什么会这样？

Answer 1

通过消除 std::integral_constant 可以获得更简单的再现:

#include <cstdint>
#include <typeinfo>
#include <cstdio>

template <typename T, T v>
struct integral_constant { };

template <typename T, T t>
constexpr integral_constant<decltype(~t), (~t)>
operator ~(integral_constant<T, t>) { return {}; }


int main() {
    constexpr auto y = integral_constant<uint8_t, 10>{};
    puts(typeid(~y).name());
}

现在fails to compile with :

<source>: In function 'int main()':
<source>:17:17: error: no match for 'operator~' (operand type is 'const integral_constant<unsigned char, 10u>')
     puts(typeid(~y).name());
                 ^
<source>:17:17: note: candidate is:
<source>:11:1: note: template<class T, T t> constexpr integral_constant<decltype (~ t), (~ t)> operator~(integral_constant<T, t>)
 operator ~(integral_constant<T, t>)
 ^
<source>:11:1: note:   template argument deduction/substitution failed:
<source>: In substitution of 'template<class T, T t> constexpr integral_constant<decltype (~ t), (~ t)> operator~(integral_constant<T, t>) [with T = unsigned char; T t = 10u]':
<source>:17:18:   required from here
<source>:11:1: error: 't' was not declared in this scope
Compiler returned: 1

看来这可以通过添加额外的括号来解决，

-integral_constant<decltype(~t), (~t)>
+integral_constant<decltype((~t)), (~t)>