Polymorphic specialisation has led to a number of hard to diagnose incorrect runtime result bugs (see #23469, #23109, #21229, #23445) so we introduced a flag `-fpolymorphic-specialisation` which allows users to turn on this experimental optimisation if they are willing to buy into things going very wrong. ``` commit 9f01d14b5bc1c73828b2b061206c45b84353620e Author: Matthew Pickering <matthewtpickering@gmail.com> Date: Mon Jun 26 10:37:55 2023 +0100 Add -fpolymorphic-specialisation flag (off by default at all optimisation levels) Polymorphic specialisation has led to a number of hard to diagnose incorrect runtime result bugs (see #23469, #23109, #21229, #23445) so this commit introduces a flag `-fpolymorhphic-specialisation` which allows users to turn on this experimental optimisation if they are willing to buy into things going very wrong. ``` But polymorphic specialisation is a Very Good Thing: see `Note [Specialising polymorphic dictionaries]` in Specialise.hs This ticket tracks progress on switching it on by-default again. We have a MR in progress, !10479, that allegedly fixes the unsound-ness. Just need to get it landed! It tickles some Simplifier infelicities, which in turn leasds to compile-time regressions, so I'm fixing them first: #23745, #23627. Things to check * Check that the regressions reported in #23791 go away. * #23109 * #23469 * #24606 * #25720 * #21229