Partial support for is_a? with exhaustiveness

[jez]

Input

→ View on sorbet.run

# typed: true

class A
  extend T::Sig
  sig {params(foo: T.any(Integer, String)).returns(Integer)}
  def bar(foo)
    if foo.is_a?(Integer)
      foo
    elsif foo.is_a?(String)
      foo.to_i
    end
  end
end

Observed output

editor.rb:6: Returning value that does not conform to method result type https://sorbet.org/docs/error-reference#7005
     6 |  def bar(foo)
     7 |    if foo.is_a?(Integer)
     8 |      foo
     9 |    elsif foo.is_a?(String)
    10 |      foo.to_i
    11 |    end
    12 |  end
  Expected Integer
    editor.rb:6: Method bar has return type Integer
     6 |  def bar(foo)
          ^^^^^^^^^^^^
  Got T.nilable(Integer) originating from:
    editor.rb:6:
     6 |  def bar(foo)
          ^^^^^^^^^^^^
    editor.rb:6:
     6 |  def bar(foo)
                  ^^^
    editor.rb:7:
     7 |    if foo.is_a?(Integer)
               ^^^^^^^^^^^^^^^^^^
    editor.rb:10:
    10 |      foo.to_i
              ^^^^^^^^
Errors: 1

Expected behavior

Since we've exhausted all the cases, it should be fine to return from this method (the inferred method return type should not include NilClass.

This can be solved the same way as #59 (see #743).