Failure in MacOS builds

[oliverlee]

MacOS builds are failing due to an ambiguous reference to 'hash'. Is there a reason for the forward declaration on this line:
https://github.com/martinmoene/type-lite/blob/master/include/nonstd/type.hpp#L747
given that type_HAVE_STD_HASH is true?

[oliverlee]

Thanks!

[martinmoene]

You're welcome :)