Rhss of code dispatching on overlapping structs are no longer
considered dead, and as such, are now typechecked.
Had to fix a test that passed only because some not-really-dead code
was not being typechecked.
(cherry picked from commit 4d5b50dee9e04aee167a7e04fbbe23526131fcad)
original commit: e0614cfed24dfc9e7f89b9a8c77e7930695b0269