range.c: support to make beginless arithmetic sequences

* range.c (range_step): fix the guard condition so that a beginless
  range can be turned into a beginless arithmetic sequence.

* test/ruby/test_range.rb (test_step): add assertions for the above
  change.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67433 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

range.c

@@ -397,7 +397,9 @@ range_step(int argc, VALUE *argv, VALUE range)
     step = (!rb_check_arity(argc, 0, 1) ? INT2FIX(1) : argv[0]);
 
     if (!rb_block_given_p()) {
-        if (rb_obj_is_kind_of(b, rb_cNumeric) && (NIL_P(e) || rb_obj_is_kind_of(e, rb_cNumeric))) {
+        const VALUE b_num_p = rb_obj_is_kind_of(b, rb_cNumeric);
+        const VALUE e_num_p = rb_obj_is_kind_of(e, rb_cNumeric);
+        if ((b_num_p && (NIL_P(e) || e_num_p)) || (NIL_P(b) && e_num_p)) {
             return rb_arith_seq_new(range, ID2SYM(rb_frame_this_func()), argc, argv,
                     range_step_size, b, e, step, EXCL(range));
         }

test/ruby/test_range.rb

@@ -226,6 +226,8 @@ class TestRange < Test::Unit::TestCase
     assert_kind_of(Enumerator::ArithmeticSequence, (0..10).step(2))
     assert_kind_of(Enumerator::ArithmeticSequence, (0..10).step(0.5))
     assert_kind_of(Enumerator::ArithmeticSequence, (10..0).step(-1))
+    assert_kind_of(Enumerator::ArithmeticSequence, (..10).step(2))
+    assert_kind_of(Enumerator::ArithmeticSequence, (1..).step(2))
 
     assert_raise(ArgumentError) { (0..10).step(0) { } }
     assert_raise(ArgumentError) { (0..).step(-1) { } }