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data A a where
  X :: A [xxx]

pattern X' = X

the inferred type of which is

>>> :info X'
pattern X' :: () => t GHC.Prim.~# [xxx] => A t

Casts use unlifted equality as I understand it, but lifted would present a nicer UI:

pattern X' :: () => t ~> [xxx] => A t
pattern X' = X

In my own code this means I get the following monster

pattern MkArr :: () => (GetTy a, GetTy b, GetTy t,
                        a GHC.Prim.~# 'Sca ('Number 'I32), b GHC.Prim.~# 'Sca a1,
                        t GHC.Prim.~# 'Arr a1, GetSca a1) => Exp a -> Id -> Exp b -> Exp t

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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