What?
We’ve made a terrible mistake when updating qt5 to 5.15 - we didn’t make sure that all applications in Nixpkgs are using the same qt versions in all inputs, and the inputs’ inputs and so on. This caused packages to appear as broken only in runtime, causing users to have segfaulting packages after their updates. The attempt to fix this is:
https://github.com/NixOS/nixpkgs/pull/101369
Explanation
The docs tell you that libsForQt5 makes sure the same qt versions are used in the inputs. Well, unfortunately without #101369 this is not correct, due to some packages inherited in the broader scope, outside the scopes of all libsForQt5* attribute sets.
Notably, the kdeApplications and kdeFrameworks are currently using qt514 by default, causing all kinds of reasons for mismatched versions to be used together.
How Can I help?
The 1st comment in #101369 lists packages affected by the change. All of them are supposed to be using the same qt versions in all inputs and their inputs, (there’s a script that helped me verify that). However, their runtime wasn’t tested, and all of these packages require a build, so your testing will be appreciated!