prevent compile time constant folding of non-portable string comparisons (e.g....

prevent compile time constant folding of non-portable string comparisons (e.g. between bytes and str)

Cython/Compiler/ExprNodes.py

@@ -44,6 +44,12 @@ try:
 except ImportError:
     basestring = str # Python 3
 
+try:
+    from builtins import bytes
+except ImportError:
+    bytes = str # Python 2
+
+
 class NotConstant(object):
     _obj = None
 
@@ -1384,7 +1390,9 @@ class StringNode(PyConstNode):
     unicode_value = None
 
     def calculate_constant_result(self):
-        self.constant_result = self.value
+        if self.unicode_value is not None:
+            # only the Unicode value is portable across Py2/3
+            self.constant_result = self.unicode_value
 
     def as_sliced_node(self, start, stop, step=None):
         value = type(self.value)(self.value[start:stop:step])
@@ -9635,6 +9643,11 @@ class CmpNode(object):
     def calculate_cascaded_constant_result(self, operand1_result):
         func = compile_time_binary_operators[self.operator]
         operand2_result = self.operand2.constant_result
+        if (isinstance(operand1_result, (bytes, unicode)) and
+                isinstance(operand2_result, (bytes, unicode)) and
+                type(operand1_result) != type(operand2_result)):
+            # string comparison of different types isn't portable
+            return
         result = func(operand1_result, operand2_result)
         if self.cascade:
             self.cascade.calculate_cascaded_constant_result(operand2_result)

tests/run/string_comparison.pyx

 
+cimport cython
+
+import sys
+IS_PY3 = sys.version_info[0] >= 3
+
 bstring1 = b"abcdefg"
 bstring2 = b"1234567"
 
@@ -211,3 +216,18 @@ def bytes_cascade_untyped_end(bytes s1, bytes s2):
     False
     """
     return s1 == s2 == b"abcdefg" == (<object>bstring1) == bstring1
+
+
+@cython.test_assert_path_exists(
+    '//CondExprNode',
+    '//CondExprNode//PrimaryCmpNode',
+    '//CondExprNode//PrimaryCmpNode[@operator = "=="]',
+    '//CondExprNode//PrimaryCmpNode[@operator = "!="]',
+)
+def literal_compare_bytes_str():
+    """
+    >>> literal_compare_bytes_str()
+    True
+    """
+    # we must not constant fold the subexpressions as the result is Py2/3 sensitive
+    return b'abc' != 'abc' if IS_PY3 else b'abc' == 'abc'