I have rebased this to current master and added the change from #95772 to Qt 5.15 as well.

This should probably go to the staging branch, can you change that?

I've rebased it to staging.

I moved the milestone. I want to have this in NixOS 20.09 because the support window for Qt 5.14 will end before NixOS 21.03. I'm building this locally right now, but obviously I won't be able to finish reviewing until tomorrow. 😅

I still want to merge this, but we will not make Qt 5.15 the default version. It is not officially supported by many packages yet, particularly it is unsupported by Plasma.

@ttuegel So we have to use a version of qt in stable for plasma that won't be supported (at some tiny point before 21.03)?

From looking at all-packages.nix, default for linux is 5.14 right now, which will be supported until 2020-12-12.
5.12 is still supported until 2020-12-12.

So we have to use a version of qt in stable for plasma that won't be supported (at some tiny point before 21.03)?

@worldofpeace We actually need to revert to Qt 5.12 in stable. Plasma 5.18 doesn't support Qt 5.14, and Plasma 5.19 is only supported until October (and isn't merged anyway).

@worldofpeace We actually need to revert to Qt 5.12 in stable. Plasma 5.18 doesn't support Qt 5.14, and Plasma 5.19 is only supported until October (and isn't merged anyway).

I don't think we should stick with LTS versions of Plasma if that means we can't upgrade Qt. AFAICT Plasma LTS releases only come around once every 1.5 years, so we would only be able to upgrade Qt every 1.5 years either.

If maintaining non-LTS versions (5.19) of Plasma is infeasible, could we instead override the Qt version only for Plasma and keep the system version at 5.15?

BTW, did you run into any other build errors with 5.15 aside from Plasma?

I don't think we should stick with LTS versions of Plasma if that means we can't upgrade Qt.

Plasma 5.18 is the only version of Plasma that will have support through our support window for NixOS 20.09, so I don't think we have much choice there. Plasma 5.20 won't be released until October, and Plasma 5.19's support window expires shortly after. We don't have the resources to be backporting security patches onto Plasma 5.19 for six months. Qt 5.12 is also an LTS release, with security updates beyond the support window of NixOS 20.09.

The status of different version combinations is:

1. I can't even build Qt 5.15 right now, let alone build anything with it.
2. Plasma 5.18 officially supports Qt 5.12. Plasma 5.18 builds with Qt 5.14, but it's not officially supported upstream and it's very glitchy. (I'm using it right now, but I haven't been able to update my main work machine because it's so bad.)
3. Plasma 5.19 supports Qt 5.14 officially. It does not support Qt 5.15, so I expect that combination to be buggy. The support window ends when Plasma 5.20 is released in October. It's too late to update for NixOS 20.09 now, and even if I begged for an exception (which I won't) I can't see how we can support it until NixOS 21.03.

I can't imagine foisting any combination other than Plasma 5.18 + Qt 5.12 on users of NixOS 20.09. The combination is supported upstream and each has LTS support through our support window. I think anything else would be an utter disaster.

Edit:

If maintaining non-LTS versions (5.19) of Plasma is infeasible, could we instead override the Qt version only for Plasma and keep the system version at 5.15?

Maybe. I think we have several custom patches to support Qt 5.14 with various applications, and even though they build, I'm not sure they work right. In general, open source applications are very slow to adapt to new Qt versions, so I don't see any reason we should be so eager. At least, I'd be happier adding the new version and waiting for application maintainers to pin their packages to the new version.

@ttuegel All of that sounds good to me. I encourage any option that makes Plasma use Qt 5.12.

I'm going to open two separate pull requests:

1. Add Qt 5.15 and make it the default for everything except Plasma and KDE Applications, which stay at Qt 5.14. (I know, yet another pull request, I'm sorry.) I think this one can go directly into master because it shouldn't be a mass rebuild.
2. Revert Plasma 5.18 to Qt 5.12. This needs to go into staging because it is a mass rebuild. This does not need to hold up the release branch because we can always push it into the release staging branch after. (It's a 3-line change, there won't be conflicts.)

I think this meets everyone's needs, and we can get it done quickly.