See also * #21229 * #23445 * #23469 * #23866 * #23923 <--------- Apparently this is the one to fix first * #24606 * #24765 (lack of polymorphic specialisation) * #20897 * #25804 * https://github.com/haskell/core-libraries-committee/issues/3 proposal to make `Eq` a unary class. * #26416 ## Original report I'd like to ask for help in what is most probably a case of a bad port (by yours truly) of a typing plugin (GHC.TypeLits.Normalise) that fails in GHC 9.6.1, but worked fine in GHC 9.4.4 and 9.2.7 (and works fine in 9.6.1 with -O0). The typing breakage is described below. The bad port to GHC 9.6 is in https://github.com/clash-lang/ghc-tcplugins-extra/pull/24 and https://github.com/clash-lang/ghc-typelits-natnormalise/pull/72. I tried to follow the migration guide, but I didn't understand much and just randomly wibbled until it type-checked. To reproduce, clone https://github.com/Mikolaj/horde-ad/tree/repro-rank-mismatch and run (the linters are included only to show they don't produce any info at all for the part of the codebase that is being compiled) cabal test simplifiedOnlyTest --test-options='-p 3concatBuild' --allow-newer --enable-optimization --ghc-options='-dcore-lint -dstg-lint -dcmm-lint -dtag-inference-checks' Normally it should pass fine, but instead you would be getting the following error Exception: fromVector: rank mismatch (1,32) where the value 32 comes from type variable `n`, from a *different* test than the one being run. If you change number 33 in this other test, you get a different error message. If you remove the other test, the error disappears. With -O0 and with older GHCs the problem does not appear. In larger versions of this reproducer, the error differed when run with profiling mode and without. If it helps, I can try to minimize further, but the corruption is very fragile and tiny changes eliminate the error or at least change it to a form that is not as obviously wrong (does not mention a number from the code it should not depend on). cc: @christiaanb