This doesn't work as advertized (empty interface type should match anything but it matches nothing):

interface any with end;

Also, if a non-empty interface type has no implementation at the time it is defined, then pure -w gives a warning about an incomplete interface type. We should just silenty treat this as an empty type. Example:

interface bar with foo x::bar; end;

This issue was closed by revision 23e242c0b321.