panic compiling file with unboxed tuples, ghc 8.2-8.4

Summary

ghc panic compiling file with unboxed tuples

Steps to reproduce

MyModule.hs

With ghc 8.2.1, 8.2.2, 8.4.3, or 8.4.4 run:

$ ghc -O1 MyModule.hs

This is minimal as I'm able to get it -- sorry I don't know how to simplify it further. It's a minimal-ized version of code from unordered-containers version 0.2.10.0 and hashable version 1.3.0.0 (the most recent version of each). I ran into the problem while trying to compile mono-traversable version 1.0.15.1.

No panic occurs with optimization flag -O0, but it does for -O2.

Expected behavior

Compile, and not panic.

Actual behaviour: I get the following output

[1 of 1] Compiling MyModule         ( MyModule.hs, MyModule.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.4 for x86_64-unknown-linux):
	isUnliftedType
  r_s44t :: TYPE rep_s44s
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1939:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Environment

• GHC version used: 8.4.4

Optional:

• Operating System: Ubuntu 16.04
• System Architecture: x86_64