Emit error when a const cypclass reference requires a write lock

Cython/Compiler/CypclassTransforms.py

@@ -542,6 +542,9 @@ class CypclassLockTransform(Visitor.EnvTransform):
             return written_node
         ref_id = self.reference_identifier(written_node)
         if ref_id:
+            if ref_id.type.is_const:
+                error(written_node.pos, "Reference '%s' is const but requires a write lock" % self.id_to_name(ref_id))
+                return written_node
             if not self.wlocked[ref_id] > 0:
                 if lock_mode == "checklock":
                     error(written_node.pos, (

tests/errors/cypclass_lock_error.pyx

@@ -32,6 +32,13 @@ cdef void global_lock_taking():
     with wlocked global_cyobject:
         global_cyobject.setter(global_cyobject.getter() + 1)
 
+
+cdef take_non_const(A a):
+    pass
+
+cdef take_const(const A a):
+    take_non_const(a)
+
 _ERRORS = u"""
 20:4: Reference 'obj' is not correctly locked in this expression (write lock required)
 21:4: Reference 'obj' is not correctly locked in this expression (read lock required)
@@ -40,4 +47,5 @@ _ERRORS = u"""
 25:4: Reference 'obj' is not correctly locked in this expression (read lock required)
 26:21: Reference 'obj' is not correctly locked in this expression (read lock required)
 32:17: Can only lock local variables or arguments
+40:19: Reference 'a' is const but requires a write lock
 """