Patrick Steinhardt [off-list ref] writes:
The question is whether it really makes sense to have tests for every
single edge case. In a perfect world we of course would, but in the real
world there are a) gazillions of different combinations and b) every
test brings its own overhead as it increases both wall time and
maintenance costs.
Very true. There needs a very good justification to add overhead to
protect what has been working fine for a long time ;-).