GADTs in Haskell98 style

it would be nice if it is possible to write GADTs in Haskell98 style, like this:

data Expr a = I Int   :: Expr Int
            | B Bool  :: Expr Bool         
            | Add (Expr Int) (Expr Int)  :: Expr Int
            | Mul (Expr Int) (Expr Int)  :: Expr Int
            | Eq  (Expr Int) (Expr Int)  :: Expr Bool 
            | Magic  (Expr a) (Expr a)  

Trac metadata

Trac field	Value
Version	7.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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