I wonder if we could make “Phase 4” be a language extension that gets added to a future GHCXXX set. I’m not sure if this is feasible, but it would be very nice in terms of a smooth transition, since it would allow old lawful code to compile indefinitely. Ofc it also comes with a maintenance cost for GHC devs. I guess it depends on how much code uses this stuff and how much of it is still maintained.
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