One possible workaround for 7.8.1 bug.

Hopefully this is not extremely intrusive.

The other alternative I propose is to define a CPP macro `SAFE_SYNONYM(synonym, expanded type, kind)` which is a closed type family in 7.8.* and a regular synonym otherwise. The problem with that solution is that it will still be in effect in 7.8.2, at which point this bug should be fixed.