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

array.c

@@ -6943,8 +6943,6 @@ flatten(VALUE ary, int level)
     }
     if (i == RARRAY_LEN(ary)) {
         return ary;
-    } else if (tmp == ary) {
-        rb_raise(rb_eArgError, "tried to flatten recursive array");
     }
 
     result = ary_new(0, RARRAY_LEN(ary));
@@ -6955,12 +6953,14 @@ flatten(VALUE ary, int level)
     rb_ary_push(stack, ary);
     rb_ary_push(stack, LONG2NUM(i + 1));
 
+    if (level < 0) {
 	vmemo = rb_hash_new();
 	RBASIC_CLEAR_CLASS(vmemo);
 	memo = st_init_numtable();
 	rb_hash_st_table_set(vmemo, memo);
 	st_insert(memo, (st_data_t)ary, (st_data_t)Qtrue);
 	st_insert(memo, (st_data_t)tmp, (st_data_t)Qtrue);
+    }
 
     ary = tmp;
     i = 0;
@@ -6974,20 +6974,24 @@ flatten(VALUE ary, int level)
 	    }
 	    tmp = rb_check_array_type(elt);
 	    if (RBASIC(result)->klass) {
+		if (level < 0) {
 		    RB_GC_GUARD(vmemo);
 		    st_clear(memo);
+		}
 		rb_raise(rb_eRuntimeError, "flatten reentered");
 	    }
 	    if (NIL_P(tmp)) {
 		rb_ary_push(result, elt);
 	    }
 	    else {
+		if (level < 0) {
 		    id = (st_data_t)tmp;
 		    if (st_is_member(memo, id)) {
 			st_clear(memo);
 			rb_raise(rb_eArgError, "tried to flatten recursive array");
 		    }
 		    st_insert(memo, id, (st_data_t)Qtrue);
+		}
 		rb_ary_push(stack, ary);
 		rb_ary_push(stack, LONG2NUM(i));
 		ary = tmp;
@@ -6997,14 +7001,18 @@ flatten(VALUE ary, int level)
 	if (RARRAY_LEN(stack) == 0) {
 	    break;
 	}
+	if (level < 0) {
 	    id = (st_data_t)ary;
 	    st_delete(memo, &id, 0);
+	}
 	tmp = rb_ary_pop(stack);
 	i = NUM2LONG(tmp);
 	ary = rb_ary_pop(stack);
     }
 
+    if (level < 0) {
 	st_clear(memo);
+    }
 
     RBASIC_SET_CLASS(result, rb_obj_class(ary));
     return result;

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 ]
@@ -2649,9 +2660,6 @@ 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) }