from the output of -ddump-splices I dont think it is the case but I'm asking anyway: is there any way to deduce a ~ b from a :==: b? 

Given

  data T = C1 | ... | Cn

I can easily derive

  data EqT :: T -> T -> * where

     EqT :: a ~ b => EqT a b

  eqT :: ST a -> ST b -> Maybe (EqT a b)

  eqT SC1 SC1 = Just EqT

  ...

  eqT SCn SCn = Just EqT

  eqT _ _ = Nothing

but this kind of replicates the boilerplate generated by the singletons library for %==%. However I can't see how to leverage %==% to inhabit eqT since I can't deduce a ~ b from a %==% b == STrue.

Any idea? If there's no way to write eqT using what singletons generates, wouldn't it make sense for it to generate something that relates :==: and ~ ?

Cheers,

Paul