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type family Blah a 
type instance Blah Int = Int
data E where
  E :: (Blah a ~ Int) => a -> E

*Example Unsafe.Coerce GHC.Prim> (unsafeCoerce E) :: Any

<interactive>:1:15:
    Couldn't match type `Blah a0' with `Int'
    In the first argument of `unsafeCoerce', namely `E'
    In the expression: (unsafeCoerce E) :: Any
    In an equation for `it': it = (unsafeCoerce E) :: Any

This also doesn't work with less dangerous operators, like seq:

*Example Unsafe.Coerce GHC.Prim> (id E) `seq` ()

<interactive>:1:5:
    Couldn't match type `Blah a0' with `Int'
    In the first argument of `id', namely `E'
    In the first argument of `seq', namely `(id E)'
    In the expression: (id E) `seq` ()

Trac metadata

Trac field	Value
Version	7.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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