Backpack signatures fail to compile when referencing class associated types with defaults

Summary

Backpack signatures fail to compile when they reference class associated types with default definitions.

Steps to reproduce

Build a library that contains a class associated type along with a default definition and require an instance of that class in a backpack signature.

Example:

backpack-test.cabal:

cabal-version: 2.4
name: backpack-test
version: 0

library common
  build-depends: base
  default-language: Haskell2010
  exposed-modules: Class
  hs-source-dirs: common

library consumer
  build-depends: base, common
  default-language: Haskell2010
  signatures: Instance
  hs-source-dirs: consumer

common/Class.hs:

{-# language TypeFamilies #-}
module Class where
class C x where
  type T x
  type T x = ()

consumer/Instance.hsig:

signature Instance where
import Class
data I
instance C I

Error given:

<no location info>: error:
    The identifier R:TI does not exist in the local signature.
    (Try adding it to the export list of the hsig file.)

Note: Removing the default type T x = () results in this code compiling.

Expected behavior

I'd expect this to work.

Environment

• GHC version used: 8.8

Optional:

• Operating System: All
• System Architecture: All