Function call returning a reference is an lvalue.

Cython/Compiler/ExprNodes.py

@@ -2460,6 +2460,9 @@ class CallNode(ExprNode):
             self.analyse_c_function_call(env)
             return True
     
+    def is_lvalue(self):
+        return self.type.is_reference
+
     def nogil_check(self, env):
         func_type = self.function_type()
         if func_type.is_pyobject: