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.
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
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.
The correct list initialization semantics is
B b{0};
which compiles fine. If you write B b({0});, the gcc can't decide if call B(A) directly or create B ({0}) and then copy it with B(const B&) in the second phase. There is no priority ordering between these two options.
It's language problem, not compiler's problem. See this gcc bug report.
