sorbet and unexpected problems with &. operator and T.untyped

a sorbet problem with &.operator and t.untyped .md

live sorbet plagyround link

sorbet.rb

# typed: strict

class A
  extend T::Sig
  
  # many (nearly all?) return types from auto-generated
  # rbi for gems are T.untyped
  sig { returns T.untyped }
  def num
    3
  end
 
  # same method, but not T.untyped
  sig { returns Integer }
  def num!
    3
  end
end

extend T::Sig

sig {params(x: Integer).returns(String)}
def bar(x)
  x.to_s
end

sig { returns T.nilable A }
def maybe_a
  rand > 0.5 ? A.new : nil
end

# both have a type error: 
#   Method num (or num!) does not exist on NilClass component of T.nilable(A)
# and this makes sense, since `maybe_a` can be nil
bar maybe_a.num
bar maybe_a.num!

# no type error, even though `maybe_a` can be nil, and 
# the `&.` operator will return nil in that case.
# This should be a type error, but the T.untyped swollows it
bar maybe_a&.num

# The only way to get a type error is to avoid accidently using methods
# that return T.untyped, and for gems this means manually writing rbi
bar maybe_a&.num!