https://git.ruby-lang.org/ruby.git/commit/?id=1b1ea7b3bc

From 1b1ea7b3bc9484e6e59d716fce2965a2f39d1e3d Mon Sep 17 00:00:00 2001
From: Marc-Andre Lafortune <github@m...>
Date: Wed, 29 Jul 2020 16:59:06 -0400
Subject: Fix Array#flatten for recursive array when given positive depth [Bug
 #17092]


diff --git a/array.c b/array.c
index 345ee32..31acc57 100644
--- a/array.c
+++ b/array.c
@@ -6943,8 +6943,6 @@ flatten(VALUE ary, int level) https://github.com/ruby/ruby/blob/trunk/array.c#L6943
     }
     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) https://github.com/ruby/ruby/blob/trunk/array.c#L6953
     rb_ary_push(stack, ary);
     rb_ary_push(stack, LONG2NUM(i + 1));
 
-    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);
+    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) https://github.com/ruby/ruby/blob/trunk/array.c#L6974
 	    }
 	    tmp = rb_check_array_type(elt);
 	    if (RBASIC(result)->klass) {
-                RB_GC_GUARD(vmemo);
-                st_clear(memo);
+		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 {
-		id = (st_data_t)tmp;
-		if (st_is_member(memo, id)) {
-                    st_clear(memo);
-		    rb_raise(rb_eArgError, "tried to flatten recursive array");
+		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);
 		}
-		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) https://github.com/ruby/ruby/blob/trunk/array.c#L7001
 	if (RARRAY_LEN(stack) == 0) {
 	    break;
 	}
-	id = (st_data_t)ary;
-	st_delete(memo, &id, 0);
+	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);
     }
 
-    st_clear(memo);
+    if (level < 0) {
+	st_clear(memo);
+    }
 
     RBASIC_SET_CLASS(result, rb_obj_class(ary));
     return result;
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 46de8e0..9e36e74 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -886,6 +886,17 @@ class TestArray < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_array.rb#L886
     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 https://github.com/ruby/ruby/blob/trunk/test/ruby/test_array.rb#L2660
 
   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) }
-- 
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