Add a `visitchild()` helper method to VisitorTransform that correctly...

Add a `visitchild()` helper method to VisitorTransform that correctly reassigns the result, which is easily forgotten.

Cython/Compiler/Visitor.pxd

@@ -19,6 +19,7 @@ cdef class VisitorTransform(TreeVisitor):
     cpdef visitchildren(self, parent, attrs=*, exclude=*)
     cdef list _flatten_list(self, list orig_list)
     cdef list _select_attrs(self, attrs, exclude)
+    cpdef visitchild(self, parent, str attr, idx=*)
 
 cdef class CythonTransform(VisitorTransform):
     cdef public context

Cython/Compiler/Visitor.py

@@ -276,6 +276,15 @@ class VisitorTransform(TreeVisitor):
                     newlist.append(x)
         return newlist
 
+    def visitchild(self, parent, attr, idx=0):
+        # Helper to visit specific children from Python subclasses
+        child = getattr(parent, attr)
+        if attr is not None:
+            node = self._visitchild(child, parent, attr, idx)
+            if node is not child:
+                setattr(parent, attr, node)
+        return child
+
     def recurse_to_children(self, node):
         self._process_children(node)
         return node