Re: [Boost-users] [thread] Ambiguity in variadic wait_for_all

Subject: Re: [Boost-users] [thread] Ambiguity in variadic wait_for_all
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-02-18 17:41:27

Le 08/02/2017 à 00:09, Gavin Lambert a écrit :
> On 8/02/2017 06:51, Vicente J. Botet Escriba wrote:
>> You have made two changes IIUC, boost version and compiler version, so I
>> don't know if this is a regression in boost 1.53  or a behavior that
>> different between c++02 and C++11. I suspect that it  is the change to
>> C++11.
>
> Sorry, I gave a misleading impression there.  Both Boost versions were 
> tested with the same compiler (VS2015), so the issue was introduced by 
> the Boost upgrade only.
>
> Explicitly disabling the variadic version by defining 
> BOOST_NO_CXX11_VARIADIC_TEMPLATES also avoids the issue, so I'm pretty 
> sure it's just the introduction of that which did it.
>
>> Curiously I added enable if only for the function with two arguments,
>> but not for the others.
>
> That's correct -- only the two-argument form is ambiguous with the 
> iterator overload.
>
> Although having said that, the iterator form takes a single template 
> parameter and the future form takes two template parameters, so the 
> compiler ought to be able to tell them apart (since the template 
> parameters were explicitly provided), unless it's trying to infer the 
> second parameter.
>
>> Please, could you try to add the enable_if to the variadic template and
>> tell me if this would fix your issue?
>>
>>     template<typename F1, typename... Fs>
>>     *typename boost::enable_if<is_future_type<F1>,void>::type*
>> wait_for_all(F1& f1, Fs&... fs)
>
> Yes, making that change resolves the ambiguity and the original code 
> compiles again.
>
>
>
This will be fixed on next release.

Vicente