libc++ does not allow forward declarations of types for std:: , as it uses inline namespace to allow versioning.
Forward declarations occur in many libraries (config, fusion, interprocess). In each case I would like to just include the actual headers.
I could just #define all cases for LIBCPP, but other libraries might move to using inline namespaces in future, so I could introduce a macro?
I think that sounds a reasonable addition, yes. Do you have patches?
IMO it should be a macro that's defined when forward declaration is possible rather than when it isn't. That way the default would be the safer option. I now think it was a mistake to forward declare for unknown setups in 'container_fwd.hpp'.
I would be happy with that, except I note that (for example) boost fusion currently unconditionally forward declares std::pair, and every compiler in the regression test (except clang with libc++) passes the std_pair test. Therefore the macro would have to be added to every other compiler, else they would become less efficent.
Note that since this is std lib issue, and since there are only a few std lib's out there, it's not that much of a change to Boost.Config to add this to every std lib config except libc++ (if indeed that's what's required). As others have noted, in principle this isn't std conforming, just useful. John.