Performance/FlatMap does not work with block_pass

[splattael]

Expected behavior

Given a test file test.rb:

# frozen_string_literal: true

# 1.
p [1, 2, 3].map { |x| x.digits }.flatten(1)

# 2.
p [1, 2, 3].map(&:digits).flatten(1)

# 3.
digits = proc { |x| x.digits }
p [1, 2, 3].map(&digits).flatten(1)

I would expect RuboCop to suggest flat_map for 1., 2., and 3.

Output:

Inspecting 1 file
C

Offenses:

test.rb:3:13: C: Performance/FlatMap: Use flat_map instead of map...flatten.
p [1, 2, 3].map { |x| x.digits }.flatten(1)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test.rb:3:17: C: Style/SymbolProc: Pass &:digits as an argument to map instead of a block.
p [1, 2, 3].map { |x| x.digits }.flatten(1)
                ^^^^^^^^^^^^^^^^
test.rb:4:13: C: Performance/FlatMap: Use flat_map instead of map...flatten.
p [1, 2, 3].map(&:digits).flatten(1)
            ^^^^^^^^^^^^^^^^^^^^^^^^
test.rb:7:13: C: Performance/FlatMap: Use flat_map instead of map...flatten.
p [1, 2, 3].map(&digits).flatten(1)
            ^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 4 offenses detected

Actual behavior

RuboCop only suggests flat_map for 1.

Output:

Inspecting 1 file
C

Offenses:

test.rb:3:13: C: Performance/FlatMap: Use flat_map instead of map...flatten.
p [1, 2, 3].map { |x| x.digits }.flatten(1)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test.rb:3:17: C: Style/SymbolProc: Pass &:digits as an argument to map instead of a block.
p [1, 2, 3].map { |x| x.digits }.flatten(1)
                ^^^^^^^^^^^^^^^^

1 file inspected, 2 offenses detected

Steps to reproduce the problem

Run rubocop -C false test.rb

RuboCop version

rubocop -V
0.72.0 (using Parser 2.6.3.0, running on ruby 2.6.3 x86_64-linux)

Suggested solultion

This patch below works as expected.

diff --git a/lib/rubocop/cop/performance/flat_map.rb b/lib/rubocop/cop/performance/flat_map.rb
index c90932f71..b5b91876a 100644
--- a/lib/rubocop/cop/performance/flat_map.rb
+++ b/lib/rubocop/cop/performance/flat_map.rb
@@ -23,7 +23,14 @@ module RuboCop
                                   'multiple levels.'

         def_node_matcher :flat_map_candidate?, <<-PATTERN
-          (send (block $(send _ ${:collect :map}) ...) ${:flatten :flatten!} $...)
+          (send
+            {
+              (block $(send _ ${:collect :map}) ...)
+              $(send _ ${:collect :map} (block_pass _))
+            }
+            ${:flatten :flatten!}
+            $...
+          )
         PATTERN

         def on_send(node)