Backpack: matching newtype selectors doesn't work

unit p where
    signature A where
        newtype F a = F { mkF :: a }
unit q where
    module A where
        newtype F a = F { mkF :: a }
unit r where
    dependency p[A=q:A]

fails with

ezyang@sabre:~$ ghc-head --backpack selector.bkp -dppr-debug
[1 of 3] Processing p
[2 of 3] Processing q
  Instantiating q
  [1 of 1] Compiling A                ( q/A.hs, q/A.o )
[3 of 3] Processing r
  Instantiating r
  [1 of 1] Including p[A=q:A]
    Instantiating p[A=q:A]
    [1 of 1] Compiling A[sig]           ( p/A.hsig, p/p-HVmFlcYSefiK5n1aDP1v7x/A.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.1.20170123 for x86_64-unknown-linux):
	TcIface.find_lbl
  missing: mkF{v}
  known labels: [mkF{q:A.mkF{v rb}}]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1166:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1170:37 in ghc:Outputable
        pprPanic, called at compiler/iface/TcIface.hs:841:41 in ghc:TcIface

Trac metadata

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