## Summary Code that compiled fine on ghc 9.12.2 crashes on todays HEAD of 9.12. I get this error message: ``` [2 of 3] Compiling Obelisk.Route ( src/Obelisk/Route.hs, dist/build/Obelisk/Route.o, dist/build/Obelisk/Route.dyn_o ) <no location info>: error: panic! (the 'impossible' happened) GHC version 9.12.2.20250922: tcMatches: multiple arguments in EmptyCase ``` I applied the following fix to my code https://github.com/heilmannsoftware/obelisk/commit/e77e222a6a30dadb0ddc7e7ced0b1fb0662c4340 which solved the problem building. ## Environment * GHC version used: HEAD of GHC 9.12 as of two days ago. Concretely the version pinned here https://gitlab.haskell.org/haskell-wasm/ghc-wasm-meta/-/commit/9c33f87b30ef84766490b7104a6a26a670f81c9a * I was compiling for the wasm backend, but the problem seems unrelated to the backend. * A GHC 9.12.2 build from a few days ago didn’t have the problem. * @TerrorJack suspects: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14045 from a few weeks ago. * This would thus affect 9.12.3 * I have no setup to easily see if this reproducible with the native backend on 9.12.3-rc1, but wanted to get this bug report out quickly. * Pinging: @wz1000 as 9.12 release manager. Optional: * Operating System: nixos * System Architecture: x86_64-linux