boost::optional compilation error with boost::function and boost::fusion::vector

[Jonathan Jones <jonathan.jones@…>]

If you compile the following on g++ 4.4.5, it will fail to compile because the optional_base copy constructor and destructor are protected.

boost::optional< boost::fusion::vector1<boost::function0<void> > >

A duplication program is attached.

[Steven Watanabe]

Well that's annoying. optional_base is an implementation detail, but it looks like it's escaping while resolving overload resolution for the copy constructor of optional_base, because of the enable_if in the (converting) constructor of fusion::vector1.

[Steven Watanabe]

This particular bug appears to be fixed in gcc 4.7.

[Steven Watanabe]

I'm not quite sure where the best place to fix this is.

• is_convertible should never fail, but it's probably not feasible to fix it.
• fusion::vector1 shouldn't cause overload resolution to fail, but I don't see any way to avoid this problem, given the limitations of is_convertible.
• I could create a special case for is_convertible for boost::function, but it would be wrong in some edge cases that a compiler intrinsic (or perhaps a C++11 implementation) would be able to get right.
• That leaves making the copy constructor of optional_base public. This ignores the underlying problem that external code is making assumptions about what it can do with an arbitrary type. I really hate to hack the code like this when it isn't the source of the problem.
• And of course the final option is to modify the constructors so that optional_base is never passed to a function that requires overload resolution against T. This one is out because it's too messy.

[Jonathan Jones <jonathan.jones@…>]

For what it's worth, the local workaround we deployed was to makes the optional_base copy constructor and destructor public, but _only_ when the BOOST_IS_CONVERTIBLE macro isn't defined. When this macro _is_ defined, is seems to be implemented in terms of lower-level compiler intrinsics, which do a better job than the boost::is_convertible implementation.