Re: [boost] [optional] Changes in Boost.Optional

Subject: Re: [boost] [optional] Changes in Boost.Optional
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-09-02 09:57:03

2014-09-02 13:39 GMT+02:00 Andrzej Krzemienski <akrzemi1_at_[hidden]>:

>
>
>
> 2014-09-02 11:23 GMT+02:00 Dean Michael Berris <mikhailberis_at_[hidden]>:
>
>
>>
>>
>> > I just verified that your code example works with 1.55 on VS2010, so it
>> is
>> > a regression. I will not be able to have a look at it for the next
>> couple
>> > of days. I would like to determine why it even works on 1.55. The
>> > converting constructor from arbitrary U is also explicit in 1.55, so it
>> > works because of some sequence of conversions that I cannot figure out
>> > right now.
>> >
>> >
>> Right. My hunch is that 1.56 now has suppression for the cases where
>> references would have been deduced (as discussed in
>>
>> http://www.boost.org/doc/libs/1_56_0/libs/optional/doc/html/boost_optional/dependencies_and_portability/optional_reference_binding.html
>> ).
>> Maybe the suppression applies even for the case when the conversion
>> operator may have been useful for copy initialization.
>>
>
> However, it works fine with GCC. It looks like VC++ compiler has a bug in
> overload resolution in copy initialization. It should never consider
> explicit constructors.
>

Ok, I managed to isolate the problem. The following code does not compile
on VC++2010, although it is correct:

struct A
{
  A(int &&) {}
  A(A &&) {}

  template <typename T>
  explicit A(T&&) {}
};

struct B
{
  operator A() { return A(1); }
  operator int() { return 0; }
};

int main()
{
  A t = B();
}

The unambiguous conversion path is to use B::operator A(), but VC++ somehow
finds it ambiguous.