Steep failure for Enumerable#filter_map

.ruby-version

2.7.1

foo.rb

class Foo
  def run
    (1...9).filter_map { |n| n * 2 if n.even? }
  end

  def run2
    (1...9).filter(&:even?).map { |n| n * 2 }
  end
end

pp Foo.new.run
#=> [4, 8, 12, 16]

pp Foo.new.run2
#=> [4, 8, 12, 16]

foo.rbs

class Foo
  def run: () -> Array[Integer]
  def run2: () -> Array[Integer]
end

Gemfile

source "https://rubygems.org"

gem "steep", git: "https://github.com/soutaro/steep.git", ref: "24363d77095f9e6c5e4f14d3ddb549317ee1d43f"

Steepfile

target :lib do
  signature "sig"
  check "lib"
end

Reproduction:

1. bundle install
2. mkdir lib sig
3. mv foo.rb lib/
4. mv foo.rbs sig/
5. bundle exec steep check

lib/foo.rb:2:2: MethodBodyTypeMismatch: method=run, expected=::Array[::Integer], actual=::Array[(::Integer & nil)] (def run)
  ::Array[(::Integer & nil)] <: ::Array[::Integer]
   ::Integer <: (::Integer & nil)
    ::Integer <: nil
==> ::Integer <: nil does not hold

module Enumerable[unchecked out Elem, out Return]: _Each[Elem, Return]
  def filter_map: [U] () { (Elem arg0) -> U? } -> ::Array[U]
                | () -> ::Enumerator[Elem, Return]
end

https://github.com/ruby/rbs/blob/503594a4901b37a9aabd30870d1824324f2de712/stdlib/builtin/enumerable.rbs#L377-L378