This problem is similar to #6273 as recently added by @jwakely

The expected to be voted in paper P0792R14 suggests to
add the declaration

template<class... S> class function_ref;    // not defined

to [func.wrap.ref.class]. This declaration is redundant since the paper suggests already to add the same declaration to [functional.syn], so this should not be done. Or, if the original paper needs to be applied as suggested, this declaration should be removed afterwards through this issue here.