It's more a limitation of the current design - if the "To" template parameter is a type which can not be used as a function parameter (which is the case here)
It cannot be passed by value, but i'm quite confident that it could be passed by (const) reference.
Switching to the implementation to use the __is_convertible builtin fixes this issue BTW, but unfortunately causes other issue...
Are you talking about switching the library implementation, or calling __is_convertible from user coder? The latter option is problematic, since my problem arises inside transform iterator: We frequently access members of structs by selecting them through a transform iterator. If, however, that member has alignment, the is_convertible metafunction called by transform iterator internals breaks.