"Pattern match on GADT" error for non-GADT

It's widely known that GADTs are just generalizations of type equality and existentials. And you can declare them that way!

{-# LANGUAGE TypeFamilies, ExistentialQuantification #-}
module NonGADT where
data T a = (a ~ ()) -> T

But pattern matching on this gives a nasty error!

f :: T a -> a
f T = ()

{-
NonGADT.hs:7:3:
    A pattern match on a GADT requires -XGADTs
    In the pattern: T
    In an equation for `f': f T = ()
 -}

Trac metadata

Trac field	Value
Version	7.0.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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