Suggest adding T.absurd when it's trivially possible

[jez]

Problem

This program already exhausts all the cases:

# typed: true
class A; end
class B; end

extend T::Sig

sig {params(ab: T.any(A, B)).void}
def foo(ab)
  case ab
  when A
    puts 'A'
  when B
    puts 'B'
  end
end

→ View on sorbet.run

Proposed solution

We could add an error (or autocorrect!) to add else T.absurd(ab) to the end of the case statement.

It's unclear when we should do this, and it could potentially be pretty noisy. Off the top of my head, I would think we should do it only for case statements (not if statements) that don't have an else. We'd have to see how much of a breaking change that would be.

[jez]

There is already precedent for this; I made a change similar to this on a branch that was never pushed or reviewed when I was driving up adoption of T.absurd internally on Stripe's monorepo.

[jez]

Relates to #3634