[ 2019-03-05 Priority set to 3 after reflector discussion ]

We don't require that the Compare template parameter can be invoked with arguments of the queue's value type. It seems like something we can use Mandates: for, since it'll be ill-formed otherwise. Something like is_invocable_r_v<bool, Compare&, value_type&, value_type&>.

This might also apply to the Compare parameter for the merge and sort functions of forward_list and list.