'Untouchable' error when using type function in class constraint in rank-2 type

I noticed there are some cases that no longer compile after the recent fix for #8644 (closed), such as the following:

{-# LANGUAGE RankNTypes, FlexibleContexts, TypeFamilies #-}

import Data.Monoid

type family Id a

type instance Id a = a
--type instance Id [a] = [Id a]

foo :: (Monoid (Id String) => r) -> r
foo x = x

main :: IO ()
main = print $ foo "Hello"

Attempting to compile this on HEAD produces the same error in 'main' as reported in the earlier ticket:

    Couldn't match expected type ‛s0’ with actual type ‛[Char]’
      ‛s0’ is untouchable

However, it compiles fine if the commented-out type family instance is used instead. It also compiles fine if the type family is replaced with an ordinary type synonym:

type Id a = a

I guess that the problem is caused by equalities of the form t ~ [Char] introduced by the type family instance.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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