It seems that group-by is incorrectly annotated. For example:
(cf (group-by even? [1 2 2]))
will return
(Map Boolean (U (Val 1) (Val 2)))
However the group-by operation returns
{false [1], true [2 2]}
so I think the type should be:
(Map Boolean (Vec (U (Val 1) (Val 2))))
Using 0.2.80
Patch to solve this.
Sorry, did not see the upload button on my first comment...
Reno, I think you need to set an email for your local git.
Fixed https://github.com/clojure/core.typed/commit/78f4b5d58d712d9b67738a807aadcec641638078