merge revision(s) 1b1ea7b3bc9484e6e59d716fce2965a2f39d1e3d,b6e6807993c770c5d2e069d8741f5dadf0b38069: [Backport #17092]

	Fix Array#flatten for recursive array when given positive depth [Bug
	 #17092]

	---
	 array.c                 | 44 ++++++++++++++++++++++++++------------------
	 test/ruby/test_array.rb | 14 +++++++++++---
	 2 files changed, 37 insertions(+), 21 deletions(-)

	Initialize memo pointer and use it consistently to silence gcc 7+

	---
	 array.c | 10 +++++-----
	 1 file changed, 5 insertions(+), 5 deletions(-)

1 files changed, 11 insertions, 3 deletions

diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 476cf795f0..f2956a7fba 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -886,6 +886,17 @@ class TestArray < Test::Unit::TestCase
     assert_raise(NoMethodError, bug12738) { a.flatten.m }
   end
 
+  def test_flatten_recursive
+    a = []
+    a << a
+    assert_raise(ArgumentError) { a.flatten }
+    b = [1]; c = [2, b]; b << c
+    assert_raise(ArgumentError) { b.flatten }
+
+    assert_equal([1, 2, b], b.flatten(1))
+    assert_equal([1, 2, 1, 2, 1, c], b.flatten(4))
+  end
+
   def test_flatten!
     a1 = @cls[ 1, 2, 3]
     a2 = @cls[ 5, 6 ]
@@ -2624,9 +2635,6 @@ class TestArray < Test::Unit::TestCase
 
   def test_flatten_error
     a = []
-    a << a
-    assert_raise(ArgumentError) { a.flatten }
-
     f = [].freeze
     assert_raise(ArgumentError) { a.flatten!(1, 2) }
     assert_raise(TypeError) { a.flatten!(:foo) }