Sorbet does not statically check return types of lambda / proc arguments

[jez]

Input

→ View on sorbet.run

# typed: true
T.let(-> {0}, T.proc.returns(String))

Observed output

No errors! Great job.

Expected behavior

The return lambda returns an Integer, but the type annotation declares that it should return a String, so Sorbet should report a type error statically.

---

I'm fine calling this bug closed after only solving the return type problem.

But note that checking argument types is trickier still:

→ View on sorbet.run

# typed: true
T.let(-> (x) {T.reveal_type(x)}, T.proc.params(x: Integer).void)

Observed output

editor.rb:2: Revealed type: T.untyped https://srb.help/7014
     2 |T.let(-> (x) {T.reveal_type(x)}, T.proc.params(x: Integer).void)
                      ^^^^^^^^^^^^^^^^
  Got T.untyped originating from:
    editor.rb:2:
     2 |T.let(-> (x) {T.reveal_type(x)}, T.proc.params(x: Integer).void)
                  ^
Errors: 1

In the CFG, Sorbet infers a type for the whole proc before it sees the Cast instruction which it could seemingly use to annotate the argument types when checking the body of the lambda.

Since it comes after, it picks T.untyped for the argument types when inferring a type for the the lambda.

[jez]

This is the same issue as #4149. That one has more test cases, this one has better context.