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I have to say `not_null(v)` reads more like an assertion than a predicate, in which case `isa<T>(not_null(v))` reads like it has the exact same semantics that `isa<T>(v)` has currently—asserts that `v` is not null. I don't dispute that you can *make* it have the desired semantics, it just won't *look* that way. maybe `isaT_or_null<Foo>(v)` ? Still looks awkward but maybe