See also: * #25924 * #23469 which is [blocked on fixing this ticket](https://gitlab.haskell.org/ghc/ghc/-/issues/23469#note_656051) Edit2: please see the small reproducer by @sheaf and @MangoIV. Edit: The old plugins used in this repro don't work with newest HEAD. I've ported the repro to ghc-9.15.20251115 and to the new versions of plugins as released on Hackage. This is on branch `VTA-fail-modern`. Note that the test will likely not pass once the bug is fixed, unlike in the original repro, but as long as it doesn't segfault any more, we won. ## Summary This may be related to #23109, since the code snapshot is from a similar date, so I've added it to the list in that ticket. I haven't tried most of the other repros from the list yet with current HEAD, but current master of horde-ad compiles and runs fine at least (which also means this is not blocking me personally in any way). This test crashes at runtime. Running under gdb shows it's a SIGSEGV: ``` gdb dist-newstyle/build/x86_64-linux/ghc-9.15.20250912/horde-ad-0.1.0.0/t/simplifiedOnlyTest/build/simplifiedOnlyTest/simplifiedOnlyTest ... Thread 5 "ghc_worker" received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x7fffe7fff6c0 (LWP 442542)] 0x0000000001bb6ff8 in stg_ap_0_fast () (gdb) bt #0 0x0000000001bb6ff8 in stg_ap_0_fast () #1 0x0000000000000000 in ?? () ``` Removing `--ghc-options="-fpolymorphic-specialisation"` from the repro makes the test pass. It's possible the cause is unsafe operations in my code that happen to show their unsoundness only when more specialization happens, but I think it's not very likely. I've minimized the code minimally so far, but I'm ready to assist in any way. Linting (`-dcore-lint -dstg-lint -dcmm-lint -dtag-inference-checks`) doesn't error out. ## Steps to reproduce ``` git clone git@github.com:Mikolaj/horde-ad.git cd horde-ad git checkout VTA-fail-fast rm -f cabal.project.local rm -rf ~/.cabal/packages/head.hackage.ghc.haskell.org cabal update cabal test -w /home/mikolaj/r/ghc.13092025/ghc/_build/stage1/bin/ghc --allow-newer --enable-optimization --ghc-options="-fpolymorphic-specialisation" ``` ## Expected behavior Test passes with `--ghc-options="-fpolymorphic-specialisation"`. ## Environment * GHC version used: ghc-9.15.20250912 Optional: * Operating System: Ubuntu * System Architecture: AMD64