Fix yielding block return type restriction underscore

[straight-shoota]

There's an unintended subtle difference for yielding blocks and underscore return value.
See #10467 (comment)

This patch replaces all underscore return types in yielding block parameters to avoid potential issues with that. This is essentially a workaround for wanky compiler behaviour.

Related to #10928

[asterite]

I think this is wrong. Underscore means anything can be returned. Omission of a type means Nil. Maybe omission should mean anything, but too late.

[asterite]

I'm surprised this isn't failing. Can I fix the underlying issue instead? Block restrictions work a bit different if a block is captured or not.

[asterite]

Of course to fix this I first need to understand the problem. Could you describe what this is solving?

[asterite]

Okay, I think I understand now. A bug should have been reported instead of just a mention in the forums.

I can try fixing underscore for the NoReturn case.

[asterite]

Here we go: #10933

I think the fix should be pretty safe. It's doing less work than before if the return type is an underscore. There's probably no way this introduced a regression.

[straight-shoota]

Yes, this might be wrong as it's not the syntax we would ideally want to use.

But it makes sure things work with the current compiler semantics. Nil return type of yielding block parameters is currently not casted to Nil but passed through:

def foo(& : ->)
  yield
end

foo { 1 } # => 1

Maybe it should. We can discuss that in #10931.

[straight-shoota]

With #10933 being merged these changes are not necessary and mostly unintended when the output values of the block parameter are being used (see #10931 (comment)).

I'll create a new PR for any intended changes.