这是 != 运算符的重写候选者

Question

The rewritten candidate set is determined as follows:

For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x <=> y.

For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y <=> x.

For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y.

For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x.

For all other operators, the rewritten candidate set is empty.

考虑以下 code：

#include <iostream>
struct Data{
    bool operator ==(int c){
      return true;
    }
};
int main(){
   Data d;
   bool r = 0 != d;  // should be ill-formed
}

不知道为什么编译器能编译出代码。 IMO，表达式 0!=d 的重写候选应包括表达式 x == y 的 所有非重写候选 。我对措辞 non-rewritten candidates 的理解是，考虑除表达式 x==y 的重写候选之外的任何候选，用于操作 x != y。毕竟，子弹4是用来组成重写的候选集的。所以，第四点不应该继续适用于为原始表达式x!=y重写的x==y。在我的示例中，0!=d 的重写候选集仅包含 0==d 的候选集，不应考虑 d==0;

的任何重写候选集

这个page中的相关规定，关于不可重写的措辞，和我的解释是一样的。（不包括改写的候选人）

For the != operator ([expr.eq]), the rewritten candidates include all member, non-member, and built-in candidates for the expression x == y.

不过，继续看下一段，即：

If a rewritten operator== candidate is selected by overload resolution for an operator @, its return type shall be cv bool, and x @ y is interpreted as:

if @ is != and the selected candidate is a synthesized candidate with reversed order of parameters, !(y == x),

既然!=改写的候选项不能继续改写，怎么会有参数顺序颠倒的合成候选项呢？我不知道为什么。 !=重写候选项的规则如何解读，特别是非重写候选项的措辞和最后一条规则？

Answer 1

我怀疑这是 [over.match.oper]/3.4.4 授予的：

For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x.

相等运算符是operator==和operator!=。另请注意，“综合”与“重写”不是同义词：四组候选人分别是会员候选人、非会员候选人、内置候选和重写候选。综合候选仅属于这些集合之一（在本例中为重写候选集合）。

因此，我的理解是 x != y 的重写候选包括

x == y 的所有未重写候选，以及
y == x.

所以 y == x 的合成候选也是重写的候选。然后 [over.match.oper]/9.1 开始：

if @ is != and the selected candidate is a synthesized candidate with reversed order of parameters, !(y == x)

即如果重写的候选是上面合成的y == x候选，那么结果就是!(y == x).

这是 != 运算符的重写候选者

which are the rewritten candidates for != operator

c++

language-lawyer

c++20