Avoid freezing nested array in Array#flatten!

Merged: https://github.com/ruby/ruby/pull/18550

array.c

@@ -6916,7 +6916,7 @@ rb_ary_flatten_bang(int argc, VALUE *argv, VALUE ary)
         }
     }
 
-    if (!(mod = ARY_EMBED_P(result) && result != child)) rb_ary_freeze(result);
+    if (result != child && !(mod = ARY_EMBED_P(result))) rb_ary_freeze(result);
     rb_ary_replace(ary, result);
     if (mod) ARY_SET_EMBED_LEN(result, 0);
 

test/ruby/test_array.rb

@@ -1018,6 +1018,12 @@ class TestArray < Test::Unit::TestCase
     assert_equal(@cls[1, 2, 3, 4, 5, 6], a5)
   end
 
+  def test_flatten_bang_does_not_freeze_nested_array
+    child = []
+    [child].flatten!
+    assert_not_predicate(child, :frozen?)
+  end
+
   def test_flatten_empty!
     assert_nil(@cls[].flatten!)
     assert_equal(@cls[],