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

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 boost::enable_if::type* wait_for_all(F1& f1, Fs&... fs)

Yes, making that change resolves the ambiguity and the original code compiles again.