Convert pack unpack failure

[philderbeast]

Reproduction of the failure:

Couldn't match type: Data.UnitsOfMeasure.Internal.Pack (Data.UnitsOfMeasure.Internal.Unpack (Base "m")) with: Base "m"

[adamgundry]

Thanks for the reproducer. I think I see roughly what is going on. The problem is that the plugin sees a flattened wanted constraint Pack (Unpack fsk) ~ fsk along with the given fsk ~ Base "m". It turns the given into a substitution and applies it to the normalised wanted, but doesn't apply the substitution under the Pack type family (which becomes an inert FamAtom).

This is fixed again in 9.2, because of the removal of flattening. With a bit of thought it might be possible to fix on earlier versions too, but I'm out of time for now.

[adamgundry]

Thanks for the report! I've incorporated the test case into #93, and raised #94 to track remaining related issues, so I'll close this.