Fix compiler crash on an assertion that had better been a safety condition.

Closes #2096.

Cython/Compiler/Optimize.py

@@ -2297,10 +2297,11 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
         arg_count = len(arg_list)
         if is_unbound_method or arg_count >= 3 or not (function.is_attribute and function.is_py_attr):
             return node
+        if not function.obj.type.is_builtin_type:
+            return node
         if function.obj.type.name in ('basestring', 'type'):
             # these allow different actual types => unsafe
             return node
-        assert function.obj.type.is_builtin_type
         return ExprNodes.CachedBuiltinMethodCallNode(
             node, function.obj, attr_name, arg_list)
 

tests/run/strmethods.pyx

@@ -55,6 +55,26 @@ def str_endswith(str s, sub, start=None, stop=None):
       return s.endswith(sub, start, stop)
 
 
+def object_as_name(object):
+    """
+    >>> object_as_name('abx')
+    True
+    >>> object_as_name('abc')
+    False
+    """
+    return object.endswith("x")
+
+
+def str_as_name(str):
+    """
+    >>> str_as_name('abx')
+    True
+    >>> str_as_name('abc')
+    False
+    """
+    return str.endswith("x")
+
+
 @cython.test_assert_path_exists(
     "//SimpleCallNode",
     "//SimpleCallNode//NoneCheckNode",