Remove overambitious constant folding of comprehensions when they occur in a...

Remove overambitious constant folding of comprehensions when they occur in a boolean context. Never assume that they are empty, since the iteration might already have required side-effects.
Closes #1920.

Cython/Compiler/ExprNodes.py

@@ -7983,6 +7983,7 @@ class ComprehensionNode(ScopedExprNode):
     child_attrs = ["loop"]
 
     is_temp = True
+    constant_result = not_a_constant
 
     def infer_type(self, env):
         return self.type

tests/run/listcomp.pyx

@@ -150,6 +150,57 @@ def listcomp_const_condition_false():
     return [x*2 for x in range(3) if False]
 
 
+@cython.test_fail_if_path_exists("//IfStatNode",
+                                 "//ComprehensionAppendNode")
+@cython.test_assert_path_exists("//ComprehensionNode")
+def listcomp_const_condition_false_bool_test():
+    """
+    >>> listcomp_const_condition_false_bool_test()
+    True
+    """
+    return not [l for l in [1] if False]
+
+
+@cython.test_fail_if_path_exists("//IfStatNode",
+                                 "//ComprehensionAppendNode")
+@cython.test_assert_path_exists("//ComprehensionNode")
+def listcomp_const_condition_false_assert():
+    """
+    >>> listcomp_const_condition_false_assert()
+    """
+    assert not [l for l in [1] if False]
+
+
+@cython.test_fail_if_path_exists("//ComprehensionNode//IfStatNode",
+                                 "//ComprehensionAppendNode")
+@cython.test_assert_path_exists("//ComprehensionNode",
+                                "//IfStatNode")
+def listcomp_const_condition_false_if():
+    """
+    >>> listcomp_const_condition_false_if()
+    True
+    """
+    if not [l for l in [1] if False]:
+        return True
+    return False
+
+
+@cython.test_fail_if_path_exists("//ComprehensionNode//IfStatNode",
+                                 "//ComprehensionAppendNode")
+@cython.test_assert_path_exists("//ComprehensionNode",
+                                "//IfStatNode")
+def listcomp_const_condition_false_typed_error():
+    """
+    >>> listcomp_const_condition_false_typed_error()  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: ...
+    """
+    cdef str l
+    if not [l for l in [1] if False]:
+        return True
+    return False
+
+
 @cython.test_fail_if_path_exists("//IfStatNode")
 @cython.test_assert_path_exists("//ComprehensionNode",
                                 "//ComprehensionAppendNode")