Comment by mrkeen

Type systems are hard! Check out https://counterexamples.org/

I gave it a quick scan for Rust examples; there are 11 samples among the following pages:

  https://counterexamples.org/eventually-nothing.html
  https://counterexamples.org/mutable-matching.html
  https://counterexamples.org/selfishness.html
  https://counterexamples.org/scope-escape.html
  https://counterexamples.org/nearly-universal.html

The first thing I always look for is mutability (combined with some other tricky feature), e.g. "Now it's a Just(3):Maybe<Int>, now it's a Nothing:Maybe<Int>, now it's a Nothing:Maybe<String>."

I wonder if TFA's example could be reproduced without mutability.