Subtyping involving functions with keyword arguments causes TR to make unsound assumptions about argument types

For some reason, this typechecks.

(: f (Number [#:y Boolean] -> Number))
(define (f x #:y [y #f] #:z [z 'this-can-be-anything])
  (if y "y is truthy" x))

To be clear, this doesn't typecheck if the z argument is removed, and it doesn't typecheck if the default for y is non-false. However, in this case, TR decides that y will always be false for some reason and lets it slide.

This isn't actually a soundness problem in this example if the optimizer is turned on. Instead, it just makes the function always return x, no matter what y is.

> (f 0 #:y #t)
- : Number
0

However, if you disable the optimizer, the soundness problem manifests, as expected.

> (f 0 #:y #t)
- : Number
"y is truthy"

Start by running the reproducer with the optimizer enabled and disabled, focusing on the keyword-argument subtyping and typechecking path. The fix is done when the example no longer permits an unsound return type and a regression test covers the differing optimizer behavior.