Backpack signature matching doesn't pick up orphan instances

Repro:

unit impl where
    module A where
        data T = T
    module B(module A, module B) where
        import A
        instance Show T where
            show T = "T"

unit sig where
    signature B where
        data T = T
        instance Show T
    module App where
        import B
        app = print T

unit main where
    dependency sig[B=impl:B]
    module Main where
        import App
        main = app

I get:

ezyang@sabre:~$ ghc-head --backpack foo.bkp
[1 of 3] Processing impl
  Instantiating impl
  [1 of 2] Compiling A                (.hs -> .o)
  [2 of 2] Compiling B                (.hs -> .o)
[2 of 3] Processing sig
[3 of 3] Processing main
  Instantiating main
  [1 of 1] Including sig[B=impl:B]
    Instantiating sig[B=impl:B]
    [1 of 2] Compiling B[sig]           (.hsig -> .o)

sig/sig-HVnmSw44WZeBfwnUur4wzl/../B.hi:1:1: error:
    No instance for (GHC.Show.Show impl:A.T)
      arising when attempting to show that
      instance [safe] GHC.Show.Show impl:A.T -- Defined in ‘impl:B’
      is provided by ‘impl:B’

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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