过载解析在gcc和clang之间得到不同的结果

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问题描述

  struct A {A（int）;}; 
 struct B {explicit B（A）; B（const B&）;}; 
 B b（{0}）; 
  
 
 
  gcc 5.1.0出现错误
  / dev / fd / 63：3：8：error：调用重载的'B（<括号括起始化列表>）'
是不明确的
 / dev / fd / 63：3：8：注意：候选人是：
 / dev / fd / 63：2：27：note：B :: B（const B&）
 / dev / fd / 63：2：21：note：B :: B（A）
  
铛铛3.6.0成功。
 
 
 哪一个是正确的？为什么？
 
 
 对于gcc 5.1.0： 
 
 
 对于clang 3.6.0： 
 
 
 这可能类似于适用。不同的是，在你的情况下， B（const B&）; 不再是Clang可接受的，因为 {0}  - & B 转换将面临两种可能性：显式构造函数或第二次递归使用复制构造函数。第一个选项，如上所述，不会被clang考虑，这次@Columbo的解释适用，并且复制构造函数不能第二次使用，因为这将需要一个用户定义的转换，因为我们有一个单一的元素（这里，  0 ）。因此，在摘要中，只有第一个构造函数成功并被采用。
 
 
 
 
 
 由于我理解问题是关于奇怪的重载分辨率规则和一些可能无法跟随，这里有一个更直观的解释。有效的规则为了
 
 
  
    b（{0}）表示goto ，然后从那里，这是我们的第一个OR上下文。枚举的两个构造函数是 B（A）; 和 B（const B&） > {0} 。 
 
 
  
  对于 B（A）转换。 
 
 
  对于 B（const B&），我们需要初始化 const B& ，其中包含，然后到（借助于否则，如果T是引用类型，T引用的类型的prvalue临时是copy-list-initialized ...），然后到。结果OR上下文具有候选 B（A）; 和 B（const B&） c $ c> 0 。这是我们的第二个OR上下文，并且是由13.3.1.7（根据over.ics.ref＃2和dcl.init.list-3的要求）的复制列表初始化。
 
 
  
  对于 B（A），构造函数是显式的，因此被Clang忽略（与规范相矛盾） （因此是模糊性）。 
 
 
  对于 B（const B&），这是@Columbo处理的场景，因此禁止将需要的用户定义转换。较新的草稿没有这个规则（但可能会被添加回来）。但是因为 0 到 const B& 是正常的用户定义转换（不是列表初始化）将忽略转换所需的显式构造函数（对于复制构造函数的这种潜在的第二次使用），因此用户定义的转换将不可能，并且该规则比我在写
 
 
 
 
 
 
 
 
 
 
 
 因此对于GCC，它可以直接使用显式构造函数，此外还可以单独使用复制构造函数。对于clang，它只考虑直接使用显式构造函数，并且它不会使用像GCC那样的副本构造函数通过副本列表初始化间接使用它。两者不会再考虑使用复制构造函数，这里不相关。
 
struct A { A(int);};
struct B { explicit B(A); B(const B&);};
B b({0});
gcc 5.1.0 gives the error
/dev/fd/63:3:8: error: call of overloaded 'B(<brace-enclosed initializer list>)'
 is ambiguous
/dev/fd/63:3:8: note: candidates are:
/dev/fd/63:2:27: note: B::B(const B&)
/dev/fd/63:2:21: note: B::B(A)
while clang 3.6.0 succeeds.
Which one is right? Why?
For gcc 5.1.0: http://melpon.org/wandbox/permlink/pVe9eyXgu26NEX6X
For clang 3.6.0: http://melpon.org/wandbox/permlink/WOi1md2dc519SPW0
This may be similar to Direct list initialization compiles successfully, but normal direct initialization fails, why? which gcc and clang get same result.
But this is a different question. B(A) is explicit here. gcc and clang get different results.
 解决方案 
The difference can be reduced to 
struct A { explicit A(int); };
struct B { B(int); };
void f(A);
void f(B);

int main() {
    f({ 1 });
}
On GCC this fails, in accordance to the Standard (which says that for list initialization, explicit constructors are considered - so they can yield an ambiguity - but they just are not allowed to be selected). Clang accepts it and calls the second function.
In your case, what @Columbo says in his answer to Direct list initialization compiles successfully, but normal direct initialization fails, why? applies. With the difference that in your case, B(const B&); is not anymore acceptable to Clang because the {0} -> B conversion would be faced with two possibilities: the explicit constructor or using the copy constructor recursively a second time. The first option, as explained above, will not be considered by clang and this time the explanation by @Columbo applies and the copy constructor cannot be used a second time because that would need a user defined conversion as we have a single element (here, 0). So in the summary, only the first constructor succeeds and is taken.
Since I understand the issue is about weird overload resolution rules and some might not be able to follow, here's a more intuitive explanation. The rules that are active are, in order 
b({0}) means goto http://eel.is/c++draft/dcl.init#17 and from there to http://eel.is/c++draft/over.match.ctor which is our first OR context . The two constructors enumerated are  B(A); and B(const B&) with argument {0}. 
For B(A) it works with a single user defined conversion. 
For B(const B&), we need to initialize a const B& which brings us to http://eel.is/c++draft/over.ics.list#8 then to http://eel.is/c++draft/over.ics.ref#2 (by help of http://eel.is/c++draft/dcl.init#dcl.init.list-3 "Otherwise, if T is a reference type, a prvalue temporary of the type referenced by T is copy-list-initialized ...") then to http://eel.is/c++draft/over.best.ics#over.ics.list-6. The resulting OR context has candidates B(A); and B(const B&), with the argument 0. This is our second OR context and is copy-list-initialization by 13.3.1.7 (as required by over.ics.ref#2 and dcl.init.list-3).
For B(A), the constructor is explicit and therefore ignored by Clang (in contradiction with the spec) but accepted by GCC (hence the ambiguity). 
For B(const B&), this is the scenario handled by @Columbo and therefore the user-defined conversion which would be needed is forbidden. Newer drafts don't have this rule anymore (but probably it will be added back). But because the 0 to const B& would be a normal user-defined conversion (not a list initialization), it would ignore the explicit constructor needed for the conversion anyway (for this potential second use of the copy constructor), and therefore the user-defined conversion wouldn't be possible anyway and the rule is of much less significance than I thought when I wrote the above shorter summary.
Therefore for GCC it can use the explicit constructor directly, and in addition by a single use of the copy constructor. For clang, it only considers using the explicit constructor directly, and it won't use it indirectly by a copy-list-initialization using the copy constructor like GCC does. Both won't consider using the copy constructor a second time, and it's irrelevant here.
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