Clang不能使用括号初始化来进行用户定义的转换 [英] Clang cannot use braced initialization for user-defined conversion
问题描述
代码如下.
struct A {
A() {}
};
struct B {
B() {}
explicit operator A() { return A{}; }
};
struct C {
A a;
C(B b) : a{b} {}
};
我有一个 struct A ，它不能聚合构造(因为它定义了一个构造函数). struct B 也是如此.但是，它也具有用于 struct A 的明确的用户定义转换运算符.现在， struct C 的构造函数采用一个 struct B ，并使用它来构造 struct A .与 cppreference 一样，转换操作员可以参与直接初始化，我相信这是 struct C 的成员初始化的情况.它通过了GCC 5.2(C ++ 11).但是，它在Clang 3.6上失败.我尝试使用C ++ 11，C ++ 14和C ++ 1z.
I have a struct A which is not aggregate constructible (because it has a constructor defined). The same goes for struct B. But it also has an explicit user-defined conversion operator to struct A. Now struct C's constructor takes a struct B, and uses it to construct struct A. As on cppreference, the conversion operator can participate in direct initialization, which I believe is the case for the member initialization of struct C. It passes on GCC 5.2 (C++11). But however it fails on Clang 3.6. I tried with C++11, C++14, and C++1z.
如果我将 a {b} 更改为 a(b)，它将同时传递Clang和GCC.
If I change a{b} to a(b), it passes on both Clang and GCC.
我想知道这是Clang错误还是我误解了标准?
I wonder if it is a Clang bug or I misunderstood the standard?
推荐答案
最新版本的gcc和clang都做对了:编译代码没有错误.
Recent version of gcc and clang both do the right thing: compile your code without errors.
在成员初始值设定项列表的第一轮重载解析中，A的构造函数不是初始值设定项列表构造函数，因此将其丢弃.
In the first round of overload resolution for the member initializer list A's constructors are discarded since they're not initializer-list constructors.
随后 [over.match.list]/p1 接管
- 如果未找到可行的初始化器列表构造函数，则重载解析再次执行，候选功能全部是T类和参数列表的构造函数由初始化列表的元素.
这一次将A的构造函数考虑在内.A的copy和move构造函数具有足够的参数以匹配重载解析过程，并且自13.3.1.3甚至13.3.1.7起，它们都不会导致任何用户定义的转换抑制(仍在
and this time A's constructors are kept into account. A's copy and move constructors have enough arguments to match the overload resolution process and they don't cause any user-defined conversion suppression since 13.3.1.3 and even 13.3.1.7 (still in Clang's code) don't apply to these constructors.
因此，将它们输入到过载解决方案集中，并应用 [over.best.ics] .
Therefore they're entered in the overload resolution set and [over.best.ics] applies.
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