StrictData and TypeFamilies regression

The credit goes to wuzzeb for originally discovering this bug here. I've minimized their test case slightly below:

{-# LANGUAGE StrictData #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

data family T
newtype instance T = MkT Int deriving Eq

With optimization enabled, this program compiles with GHC 8.0.2 through 8.4.4, but not with 8.6.3 or HEAD:

$ /opt/ghc/8.4.4/bin/ghc -fforce-recomp -O Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

$ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:6:39: error:
    • Couldn't match a lifted type with an unlifted type
        arising from the coercion of the method ‘==’
          from type ‘GHC.Prim.Int# -> GHC.Prim.Int# -> Bool’
            to type ‘T -> T -> Bool’
    • When deriving the instance for (Eq T)
  |
6 | newtype instance T = MkT Int deriving Eq
  |                                       ^^

Based on the error message, it appears as if GHC mistakenly believes that the representation type of the T instance is Int#, rather than Int.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
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