Compiler panic when handling gap-looking string with escaping

Summary

Compiler panic when compiling interpolate test suite.

Building test suite 'spec' for interpolate-0.2.1...
[ 3 of 13] Compiling Data.String.Interpolate.Internal.UtilSpec ( test/Data/String/Interpolate/Internal/UtilSpec.hs, dist-newstyle/build/x86_64-linux/ghc-9.12.1/interpolate-0.2.1/t/spec/build/spec/spec-tmp/Data/String/Interpolate/Internal/UtilSpec.o ) [Source file changed]
<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.12.1:
	gap unexpectedly ended

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


Error: [Cabal-7125]
Failed to build test:spec from interpolate-0.2.1.

Steps to reproduce

The error comes from this line https://github.com/sol/interpolate/blob/a31cd1322665b10a6d901d6d70dd7bc35b71035d/test/Data/String/Interpolate/Internal/UtilSpec.hs#L75C41-L75C43

and can be isolated to

module Data.String.Interpolate.Internal.UtilSpec where

bug :: String
bug = "\^\ "

Expected behavior

Don't panic!

Environment

GHC version used:

Observed on nixpkgs using 9.12.2 and then reproduced with cabal-install and 9.12.1

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