type narrowing for block_given?

[HoneyryderChuck]

If I have the following:

def each: () { (Integer) -> void } -> void
              | () -> ::Enumerator[Integer, void]

then the following method presents a steep type error:

def each(&)
  return enum_for(__method__) unless block_given?

  @bag_of_ints.each(&)
  # Cannot pass a value of type `(^(::Integer) -> void | nil)` as a block-pass-argument of type `^(::Integer) -> void`
  # (^(::Integer) -> void | nil) <: ^(::Integer) -> void
  #   nil <: ^(::Integer) -> void(Ruby::BlockTypeMismatch)

because the block_given? call isn't narrowing the type.

The error goes away if I defined the function with (&blk) and do unless blk instead. However, rubocop will enforce nameless blocks in autocorrect for modern ruby, which exposes this error on ruby upgrades.

[soutaro]

Hmm, implementing type narrowing based on the block_given? call might be the solution...