I had an idea for how we might be able to unpack coercions in a limited and conservative way that doesn't get into the general problems of unpacking constraints. Would this make sense?
1. Move the definition of Coercion from Data.Type.Coercion to GHC.Magic 2. Magically change the type of the Coercion constructor from Coercion :: Coercible a b => Coercion a b to Coercion :: a ~R# b => Coercion a b Now a strict Coercion field will unpack to nothing.
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