With respect to C_CPP: Return value instead of pointer in list_concat, it seems returning value is a better approach. It simplifies the logic for handling return value of list_concat, as we can see in the commit.

A value returned is also helpful in visit_ListLen():

visit_ListSection() already returns a value:

But it is not appropriately casted (using brackets) which is fixed in commit C_CPP: Fix list section in this PR.