Sorbet does not detect the non-existence of the "sig" method

[alexevanczuk]

Input

→ View on sorbet.run

# typed: strict
class A
  sig { returns(String)}
  def bar
    'test'
  end
end

A.new.bar

Observed output

No errors! Great job

Expected behavior

Sorbet should say:

editor.rb:3: Method sig does not exist on T.class_of(A) https://srb.help/7003
     3 |  sig { returns(String)}
          ^^^^^^^^^^^^^^^^^^^^^^^
    https://github.com/sorbet/sorbet/tree/master/rbi/core/kernel.rbi#L554: Did you mean: Kernel#send?
     554 |  def send(arg0, *arg1, &blk); end
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Context

Context from slack:

1. https://sorbet-ruby.slack.com/archives/CJL5B3CEN/p1589999692123900
2. https://sorbet-ruby.slack.com/archives/CHN2L03NH/p1590000596388800

[jez]

#2836 introduced this regression.

[jez]

A potential solution would be to instead translate all sigs to an intrinsic like

sig(...) {...}

# becomes

<Magic>.sig(self, ...) {...}

and then to have special handling in calls.cc to ensure that if we ever did see such a <Magic>.sig call, that sig must be a valid method on self when given the same args.

[marianosimone]

This was fixed in #3083