| A variety of tests around reachability. These tests in general check | |
| two things: | |
| - that we get unreachable code warnings in reasonable locations; | |
| - that we permit coercions **into** `!` from expressions which | |
| diverge, where an expression "diverges" if it must execute some | |
| subexpression of type `!`, or it has type `!` itself. |