Function with type error accepted

The following program is accepted by ghc even though it is clearly wrong. The type family T is closed so T Bool cannot ever be reduced, so it must be a type error.

{-# LANGUAGE TypeFamilies #-}
module Err62 where

type family T a where
    T Int = ()

x :: T Bool
x = undefined

f :: T Bool -> Bool
f _ = True

y :: Bool
y = f x

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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