See also: * #24984 ## Summary GHC 9.8.1 takes increasingly longer (and uses radically more memory) to compile a module as fields are added to a data type; the same did not occur with GHC 9.6. ## Steps to reproduce The repo at https://github.com/m4dc4p/ghc98-bug can demonstrate the issue. You can just run `cabal build` to try it. The file `AAAX.hs` contains the `HKDType` data type. As you uncomment fields, the compilation times (`cabal build`) take longer and longer. For example, on my M2, with GHC 9.8, I get these timings & peak memory usage: * 1 field - 2.5s, 198MB peak * 2 fields - 7s, 1.0GB peak * 3 fields - 26.8s, 4.6GB peak * 4 fields - 82.9s, 14.5GB peak Note that with -O0, there is no problem - compilation time does not increase. Also, removing the standalone `Eq` instance also causes the problem to go away (but, of course, I want that instance). Under GHC 9.6, with -O1, compilation times do not increase either (even up to 10 fields on the data type). I found this on MacOS 14.2.1, using an M2 chip. (Note I also reported this as a bug against the Barbies library - https://github.com/jcpetruzza/barbies/issues/51)