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

Subject: [Boost-users] [thread] Ambiguity in variadic wait_for_all
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-02-03 01:38:41

A colleague recently ran into an interesting issue when upgrading some 
working code from Boost 1.55 to 1.60/1.63 (and from C++03 to C++11).

The exact details are unimportant, but it was trying to get the address 
of boost::wait_for_all(Iterator, Iterator), for use in a Boost.Bind call.

In 1.55, this worked fine, although of course it had to supply template 
parameters that proved the parameters were not futures (thus causing 
enable_if<is_future_type<F1>> to fail, disabling the two-future-args 
overload of the function.

In 1.60/1.63, the variadic method is encountered instead, which lacks 
any enable_if condition and thus the ambiguity between the two remains 
and the compiler errors out.  (When calling it directly, the variadic 
one manages to get disabled as long as the iterator type lacks a wait() 
method, but this doesn't apply when just getting the method address, as 
it's not part of the signature.)

Should the variadic form also have enable_if?  Or is this considered 
user error for trying to use Bind in a variadic-compatible compiler 
instead of using a lambda?