simplify and generalise comprehension/genexpr handling fix for if-const optimisation

Cython/Compiler/Optimize.py

@@ -3119,26 +3119,15 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                 node.else_clause = if_clause.body
                 break
             else:
+                # False clauses can safely be deleted
                 assert condition_result == False
-                # prevent killing generator expressions,
-                # but simplify them as much as possible
-                yield_expr_stat = self._find_genexpr_yield_stat(if_clause.body)
-                if yield_expr_stat is not None:
-                    # => if False: yield None
-                    yield_expr = yield_expr_stat.expr
-                    if_clause.condition = ExprNodes.BoolNode(
-                        if_clause.condition.pos,
-                        value=False, constant_result=False)
-                    yield_expr.arg = ExprNodes.NoneNode(yield_expr.arg.pos)
-                    if_clause.body = yield_expr_stat
-                    if_clauses.append(if_clause)
-                else:
-                    # False clauses outside of generators can safely be deleted
-                    pass
-        if not if_clauses:
-            return node.else_clause
+        if if_clauses:
             node.if_clauses = if_clauses
             return node
+        elif node.else_clause:
+            return node.else_clause
+        else:
+            return Nodes.StatListNode(node.pos, stats=[])
 
     def visit_ForInStatNode(self, node):
         self.visitchildren(node)

tests/run/generators_py.py

 # mode: run
 # tag: generators
 
+import cython
+
 try:
     from builtins import next # Py3k
 except ImportError:
@@ -362,3 +364,23 @@ def test_del_in_generator():
     del x
     yield a
     del a
+
+@cython.test_fail_if_path_exists("//IfStatNode", "//PrintStatNode")
+def test_yield_in_const_conditional_false():
+    """
+    >>> list(test_yield_in_const_conditional_false())
+    []
+    """
+    if False:
+        print(yield 1)
+
+@cython.test_fail_if_path_exists("//IfStatNode")
+@cython.test_assert_path_exists("//PrintStatNode")
+def test_yield_in_const_conditional_true():
+    """
+    >>> list(test_yield_in_const_conditional_true())
+    None
+    [1]
+    """
+    if True:
+        print(yield 1)

tests/run/listcomp.pyx

@@ -87,3 +87,23 @@ def sorted_listcomp(sequence):
     [3, 4, 5]
     """
     return sorted([ n+1 for n in sequence ])
+
+@cython.test_fail_if_path_exists("//IfStatNode",
+                                 "//ComprehensionAppendNode")
+@cython.test_assert_path_exists("//ComprehensionNode")
+def listcomp_const_condition_false():
+    """
+    >>> listcomp_const_condition_false()
+    []
+    """
+    return [x*2 for x in range(3) if False]
+
+@cython.test_fail_if_path_exists("//IfStatNode")
+@cython.test_assert_path_exists("//ComprehensionNode",
+                                "//ComprehensionAppendNode")
+def listcomp_const_condition_true():
+    """
+    >>> listcomp_const_condition_true()
+    [0, 2, 4]
+    """
+    return [x*2 for x in range(3) if True]