disallow assignment from bytes to basestring (only allow str/unicode)

Cython/Compiler/ExprNodes.py

@@ -1160,7 +1160,7 @@ class BytesNode(ConstNode):
         node = BytesNode(self.pos, value=self.value,
                          constant_result=self.constant_result)
         if dst_type.is_pyobject:
-            if dst_type in (py_object_type, Builtin.bytes_type, Builtin.basestring_type):
+            if dst_type in (py_object_type, Builtin.bytes_type):
                 node.type = Builtin.bytes_type
             else:
                 self.check_for_coercion_error(dst_type, env, fail=True)

Cython/Compiler/PyrexTypes.py

@@ -963,7 +963,7 @@ class BuiltinObjectType(PyObjectType):
     def assignable_from(self, src_type):
         if isinstance(src_type, BuiltinObjectType):
             if self.name == 'basestring':
-                return src_type.name in ('bytes', 'str', 'unicode', 'basestring')
+                return src_type.name in ('str', 'unicode', 'basestring')
             else:
                 return src_type.name == self.name
         elif src_type.is_extension_type:

tests/run/builtin_basestring.pyx

@@ -60,8 +60,6 @@ def basestring_typed_variable(obj):
     assert s
     s = 'abc'
     assert s
-    s = b'abc'
-    assert s
     # make sure coercion also works in conditional expressions
     s = u'abc' if obj else b'abc' if obj else 'abc'
     assert s