Ömer Sinan Ağacan authored 5 years ago and Marge Bot committed 5 years ago

Previously if we had

    f |> co

where `f` had arity type `ABot N` and `co` had arity M and M < N,
`arityType` would return `ABot M` which is wrong, because `f` is only
known to diverge when applied to `N` args, as described in Note
[ArityType]:

    If at = ABot n, then (f x1..xn) definitely diverges. Partial
    applications to fewer than n args may *or may not* diverge.

This caused incorrect eta expansion in the simplifier, causing #16066.

We now return `ATop M` for the same expression so the simplifier can't
assume partial applications of `f |> co` is divergent.

A regression test T16066 is also added.