TemplateHaskell with StrictData and -funbox-strict-fields panics

Summary

Compiler panics with the following trivial example.

Steps to reproduce

Only required package is vector-th-unbox and code is:

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE StrictData #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -funbox-strict-fields #-}

import Data.Vector.Unboxed.Deriving (derivingUnbox)

newtype X
  = X { unX :: Word }
  deriving (Eq, Ord, Show)

data Y = Y
  { yX :: X
  , yD :: Double
  } deriving (Eq, Show)

derivingUnbox "Y"
  [t| Y -> (Word, Double) |]
  [| \ y -> (unX (yX y), yD y) |]
  [| \ (x, d) -> Y (X x) d |]

main :: IO ()
main = putStrLn "Hello!"

which results in:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.6.5 for x86_64-unknown-linux):
	mkNewTyConRhs
  MV_Y [Int#, MVector s_a5Mc[sk:1] Word, MVector s_a5Mc[sk:1] Double]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler/iface/BuildTyCl.hs:65:27 in ghc:BuildTyCl

Removing the StrictData language pragma or the -funbox-strict-fields option allows this to compile.

Expected behavior

This code should panic the compiler.

Environment

• GHC version used: 8.6.5

Optional:

• Operating System: Linux
• System Architecture: x86_64