[英]Inherited synthesized comparison operator produces warning/error: ISO C++20 considers use of overloaded operator '!='
In the following code snippet clang 11.0.1 generates a warning在以下代码片段中 clang 11.0.1 生成警告
template <class T>
struct iterator_facade
{
template<class S>
bool operator==(const S &other) const noexcept
{
return static_cast<const T &>(*this).equal_to(other);
}
};
struct iterator : public iterator_facade<iterator>
{
bool equal_to(const iterator &) const noexcept
{
return true;
}
};
bool check(iterator a, iterator b)
{
return a == b;
}
The code live: https://godbolt.org/z/65zWEq实时代码: https://godbolt.org/z/65zWEq
source>:21:14: warning: ISO C++20 considers use of overloaded operator '==' (with operand types 'iterator' and 'iterator') to be ambiguous despite there being a unique best viable function [-Wambiguous-reversed-operator]
return a == b;
~ ^ ~
<source>:5:7: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
bool operator==(const S &other) const noexcept
^
The code above compiles successfully with Visual C++ (VS 16.8.x) and with previous preview (VS 16.9.0 Preview 2).上面的代码使用 Visual C++ (VS 16.8.x) 和以前的预览版 (VS 16.9.0 Preview 2) 成功编译。 However, recently released VS 16.9.0 Preview 3 now produces error for this code snippet:
但是,最近发布的 VS 16.9.0 Preview 3 现在会为此代码片段产生错误:
1>C:\MyProjects\test\test\source.cpp(21,16): error C2666: 'foo<bar>::operator ==': 2 overloads have similar conversions
1>C:\MyProjects\test\test\source.cpp(5,7): message : could be 'bool iterator_facade<iterator>::operator ==<bar>(const S &) noexcept const' [rewritten expression '!(x == y)']
1> with
1> [
1> S=iterator
1> ]
1>C:\MyProjects\test\test\source.cpp(5,7): message : or 'bool iterator_facade<iterator>::operator ==<iterator>(const S &) noexcept const' [synthesized expression '!(y == x)']
1> with
1> [
1> S=iterator
1> ]
1>C:\MyProjects\test\test\source.cpp(21,16): message : while trying to match the argument list '(iterator, iterator)'
Does it look like there is no compliant way to provide synthesized comparison operators for a derived class iterator
with CRTP class iterator_facade
?看起来没有合规的方法可以为带有 CRTP class
iterator_facade
的派生 class iterator
提供综合比较运算符?
The issue is we have this comparison operator:问题是我们有这个比较运算符:
template<class T>
struct iterator_facade
{
template <class S>
bool operator==(const S &other) const noexcept;
};
So when we try to compare two iterator
s, we have these two candidates:所以当我们尝试比较两个
iterator
时,我们有这两个候选者:
bool operator==(iterator_facade<iterator> const&, iterator const&); // the member candidate
bool operator==(iterator const&, iterator_facade<iterator> const&); // the reversed member candidate
And the problem is that the member candidate is an exact match in the 2nd argument but a derived-to-base conversion in the first pair... and the reversed candidate is an exact match in the 1st argument but a derived-to-base conversion in the second.问题是成员候选者在第二个参数中是完全匹配的,但在第一对中是派生到基础的转换......而反转的候选者是第一个参数中的完全匹配,但是是派生到基础第二次转换。 This means neither candidate is better than the other, and the two are ambiguous.
这意味着没有一个候选人比另一个更好,而且两者都是模棱两可的。
Now, since this library actually requires C++20 to begin with, this is kind of pointless.现在,由于这个库实际上需要 C++20 开始,这有点毫无意义。 Just make
iterator
implement operator==
on its own, this adds... nothing?只需让
iterator
自行实现operator==
,这会增加......什么都没有? I'm not sure what it adds anyway.无论如何,我不确定它添加了什么。
But if you really want to make it work, you would need to provide a second overload that takes two instances of derived.但是如果你真的想让它工作,你需要提供第二个重载,它需要两个派生实例。 As in:
如:
template <class T>
struct iterator_facade
{
friend bool operator==(T const& a, T const& b) noexcept {
return a.equal_to(b);
}
template <sentinel_for<T> S>
bool operator==(S const& other) const noexcept;
};
The homogeneous operator will be a better match than the heterogeneous one, so the comparison will compile fine.同质算子将比异质算子更好地匹配,因此比较会编译得很好。
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