ci: Bump channel to nixos-26.05 #296

Mostly to get more recent iwyu coverage (ref #294 (comment))

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #288 (Create support branch for CI scripts, documentation, and examples by ryanofsky)
• #287 (Split repository into master (library source) and support (CI, docs, examples) branches. by ryanofsky)
• #274 (Add nonunix platform support by ryanofsky)
• #212 (ci: add newdeps job testing newer versions of cmake and capnproto by ryanofsky)
• #209 (cmake: Increase cmake policy version by ryanofsky)
• #163 (build: set cmake policy version to 3.31 by purpleKarrot)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Some bumps:

• gcc 14 -> 15
• llvm 20 -> 22 (albeit shell.nix pins it to 21)
• cmake 3 -> 4
• capn 1.1 -> 1.4

I wonder if it makes sense to have an llvm-olddeps check here as well (mostly for capn 1.1), to check the full range, but this can be done in a follow-up. Looks loke olddeps fails anyway ...

I am at a loss on reproducing and fixing the cmake 3.12 compile failure. Also, I don't see the point on wasting more time with ancient cmake versions that are only used by a single person, when #175 can be done, so I've included that here, to get the CI green.

Ok, the failure seems to be:

    4206-/nix/var/nix/builds/nix-37473-276334059/cmake-3.12.4/Utilities/cmcurl/lib/hostip.c: In function 'Curl_resolv_timeout':
    4207:/nix/var/nix/builds/nix-37473-276334059/cmake-3.12.4/Utilities/cmcurl/lib/hostip.c:721:23: error: assignment to '__sighandler_t' {aka 'void (*)(int)'} from incompatible pointer type 'int (*)
    (int)' [-Wincompatible-pointer-types]
    4208-  721 |     sigact.sa_handler = alarmfunc;
    4209-      |                       ^
    4210-/nix/var/nix/builds/nix-37473-276334059/cmake-3.12.4/Utilities/cmcurl/lib/hostip.c:623:12: note: 'alarmfunc' declared here
    4211-  623 | RETSIGTYPE alarmfunc(int sig)
    4212-      |            ^~~~~~~~~
    4213-In file included from /nix/var/nix/builds/nix-37473-276334059/cmake-3.12.4/Utilities/cmcurl/lib/hostip.c:46:
    4214-/nix/store/15h9askp4k1lx44d9871wid23j2a8ijp-glibc-2.42-61-dev/include/signal.h:72:16: note: '__sighandler_t' declared here
    4215-   72 | typedef void (*__sighandler_t) (int);
    4216-      |                ^~~~~~~~~~~~~~

I used NIX_PATH='nixpkgs=https://github.com/NixOS/nixpkgs/archive/bd0ff2d3eac24699c3664d5966b9ef36f388e2ca.tar.gz' nix --extra-experimental-features 'nix-command flakes' build --impure --expr 'let pkgs = import <nixpkgs> {}; in (pkgs.cmake.overrideAttrs (old: { version = "3.12.4"; src = pkgs.fetchurl { url = "https://cmake.org/files/v3.12/cmake-3.12.4.tar.gz"; hash = "sha256-UlVYS/0EPrcXViz/iULUcvHA5GecSUHYS6raqbKOMZQ="; }; patches = []; })).override { isMinimalBuild = true; }' -L

So I guess the alternative would be to pin olddeps to an older snapshot/channel, but this just seems tedious for little benefit.

Thanks for the PR. The main change fa2c56ec27f21eac9a102afadd49a073f74504f2 looks good but as discussed previously I don't think 34f48d1e8f85dcd471c16a14b0bae01c1f2d561d is a good change because it is changing the cmake policy version and minimum version at the same time. These are two different concepts that have different effects and should not be coupled together, especially without even mentioning the policy changes in the commit message or PR description.

If you believe it's important to trigger a fatal error that says cmake 3.22 is required (even though it is not required), I think you should do that without changing the policy version. Or you might consider just showing a warning that versions <= 3.22 are not supported. Or you might consider dropping the cmake change from this PR and leaving it for a dedicated PR.

I also don't think the ci.sh changes in 34f48d1e8f85dcd471c16a14b0bae01c1f2d561d are good and would prefer to expand CI coverage (#212) rather than reduce it. But if there isn't a simpler / better way to fix the nixpkg error #296 (comment) maybe those changes would be worth it. Would want to investigate a little more.

... as discussed previously I don't think 34f48d1 is a good change because it is changing the cmake policy version and minimum version at the same time.

I think the discussion about CMake policies would benefit from starting with a few basic questions:

1. Which policies affect the project?
2. Which minimum Policy Version is required by the project?

All previous changes in this repository, including 06e1045baba09fc1fb876b6cb48d93d4b41b101b, 6902bfd40e33a0007f75bce8af0fc0b10a147e86, and 729ff16d559c7727e883850c6ed245ec48d11954, do not provide details in that regard.

I don't think there is a policy in 3.12->3.22 that affects libmul.

However, if you want me to push something like this to #175, then I am happy to do that.

cmake_minimum_required(VERSION 3.22)
cmake_policy(VERSION 3.12)  # Set older policy than minimum, after minimum was bumped. This line has no effect on this project, but was done to minimize and de-tangle changes

Then, I can open a follow-up to remove the line again.