fix DECREF(NULL) for empty comprehensions

Cython/Compiler/ExprNodes.py

@@ -7896,7 +7896,7 @@ class ScopedExprNode(ExprNode):
 
         # normal (non-error) exit
         for entry in py_entries:
-            code.put_var_decref_clear(entry)
+            code.put_var_xdecref_clear(entry)
 
         # error/loop body exit points
         exit_scope = code.new_label('exit_scope')
@@ -7906,7 +7906,7 @@ class ScopedExprNode(ExprNode):
             if code.label_used(label):
                 code.put_label(label)
                 for entry in py_entries:
-                    code.put_var_decref_clear(entry)
+                    code.put_var_xdecref_clear(entry)
                 code.put_goto(old_label)
         code.put_label(exit_scope)
         code.putln('} /* exit inner scope */')

tests/run/cython3.pyx

@@ -329,6 +329,7 @@ def loop_over_unicode_literal():
         assert uchar in 'abcdefg'
     return cython.typeof(uchar)
 
+
 def list_comp():
     """
     >>> list_comp()
@@ -339,6 +340,28 @@ def list_comp():
     assert x == 'abc' # don't leak in Py3 code
     return result
 
+
+def list_comp_iterable(it):
+    """
+    >>> list_comp_iterable([])
+    []
+    >>> list_comp_iterable([0])
+    [0]
+    >>> list_comp_iterable([1])
+    []
+    >>> list_comp_iterable([0, 1])
+    [0]
+    >>> list_comp_iterable([2])
+    [4]
+    >>> list_comp_iterable(range(5))
+    [0, 4, 8]
+    """
+    x = 'abc'
+    result = [x*2 for x in it if x % 2 == 0]
+    assert x == 'abc' # don't leak in Py3 code
+    return result
+
+
 def list_comp_with_lambda():
     """
     >>> list_comp_with_lambda()

tests/run/listcomp.pyx

@@ -97,6 +97,10 @@ def listcomp_as_condition(sequence):
 @cython.test_assert_path_exists("//ComprehensionNode")
 def sorted_listcomp(sequence):
     """
+    >>> sorted_listcomp([])
+    []
+    >>> sorted_listcomp([1])
+    [2]
     >>> sorted_listcomp([3,2,4])
     [3, 4, 5]
     """