GHC HEAD regression: Template variable unbound in rewrite rule

hybrid-vectors-0.2.1 currently fails to build with GHC HEAD. I've attached a minimized repro file with no dependencies (it's 410 lines long, since I had to copy-paste a lot of code from vector in order to reproduce this). The attached file builds on GHC 7.10.3 and 8.0.2, but fails on GHC HEAD with this panic:

$ ~/Software/ghc2/inplace/bin/ghc-stage2 -O2 -fforce-recomp Bug.hs
[1 of 1] Compiling Data.Vector.Hybrid.Internal ( Bug.hs, Bug.o )

Bug.hs:389:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘gmbasicOverlaps’, ‘gmbasicInitialize’, and ‘gmbasicUnsafeRead’
    • In the instance declaration for ‘GMVector (MVector u v) (a, b)’
    |
389 | instance (GMVector u a, GMVector v b) => GMVector (MVector u v) (a, b) where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug.hs:407:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘gbasicUnsafeFreeze’, ‘gbasicUnsafeThaw’, ‘gbasicLength’,
        ‘gbasicUnsafeSlice’, and ‘gbasicUnsafeIndexM’
    • In the instance declaration for ‘GVector (Vector u v) (a, b)’
    |
407 | instance (GVector u a, GVector v b) => GVector (Vector u v) (a, b) where
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.3.20170310 for x86_64-unknown-linux):
        Template variable unbound in rewrite rule
  Variable: sc_s8FV
  Rule "SC:$j0"
  Rule bndrs: [sc_s8FV, sc_s8FW, sc_s8FX, sg_s8FY, sc_s8FU]
  LHS args: [sc_s8FU,
             (MV
                @ (GMutable u_a4oM)
                @ (GMutable v_a4oO)
                @ (PrimState (ST RealWorld))
                @ (a_a4oN, b_a4oP)
                @ a_a4oN
                @ b_a4oP
                @~ (<(a_a4oN, b_a4oP)>_N
                    :: ((a_a4oN, b_a4oP) :: *) ~# ((a_a4oN, b_a4oP) :: *))
                sc_s8FW
                sc_s8FX)
             `cast` (sg_s8FY
                     :: (MVector
                           (GMutable u_a4oM)
                           (GMutable v_a4oO)
                           (PrimState (ST RealWorld))
                           (a_a4oN, b_a4oP) :: *)
                        ~R#
                        (GMutable
                           (Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))]
  Actual args: [sc_s8FS,
                wild_X3x
                `cast` (Sub
                          (Sym (D:R:GMutableVector[0] <u_a4oM>_N <v_a4oO>_N)) <PrimState
                                                                                 (ST
                                                                                    RealWorld)>_N (Sym
                                                                                                     cobox_a4Gz)
                        :: (MVector
                              (GMutable u_a4oM)
                              (GMutable v_a4oO)
                              (PrimState (ST RealWorld))
                              (a_a4oN, b_a4oP) :: *)
                           ~R#
                           (GMutable
                              (Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))]
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable
        pprPanic, called at compiler/specialise/Rules.hs:579:19 in ghc:Rules

Some important things to note:

• -O2 is required to trigger this bug; using -O1 or below works fine.
• The problematic code is the Read (Vector u v c) instance at the very bottom. In particular, commenting out the last line readPrec = greadPrec makes the panic go away.
• Moreover, there's a particular INLINE pragma in the GMVector (MVector u v) (a, b) instance that is critical to triggering the panic (it's on line 395; I've added a comment Removing this INLINE pragma makes it compile above it).