Add clang 21 compiler tests to CI

.github/workflows/ci.yml

@@ -52,6 +52,9 @@ jobs:
          - name: clang-20
            shell: ci_clang20
            darwin: True
+         - name: clang-21
+           shell: ci_clang21
+           darwin: True
          # CPU flags are not correctly passed to the zig assembler
          # https://github.com/ziglang/zig/issues/23576
          # We therefore only test the C backend

.github/workflows/nix.yml

@@ -88,12 +88,15 @@ jobs:
       fail-fast: false
       matrix:
         target:
+          # nixpkgs requires 2.18 since August 2025, see 
+          # https://github.com/NixOS/nixpkgs/pull/428076
+          # TODO: Re-enable tests on Ubuntu 22 once nix has been updated to >= 2.18
+          # - runner: ubuntu-22.04
+          #   container:
+          #   install: 'apt'
           - runner: ubuntu-latest
-            container: nixos/nix:2.6.1
+            container: nixos/nix:2.18.0
             install: 'native'
-          - runner: ubuntu-22.04
-            container:
-            install: 'apt'
           - runner: ubuntu-24.04
             container:
             install: 'apt'

flake.nix

@@ -52,6 +52,7 @@
                 gcc48 = pkgs-2405.gcc48;
                 gcc49 = pkgs-2405.gcc49;
                 gcc7 = pkgs-2405.gcc7;
+                clang_21 = pkgs-unstable.clang_21;
               })
             ];
           };
@@ -115,6 +116,7 @@
           devShells.ci_clang18 = util.mkShellWithCC' pkgs.clang_18;
           devShells.ci_clang19 = util.mkShellWithCC' pkgs.clang_19;
           devShells.ci_clang20 = util.mkShellWithCC' pkgs.clang_20;
+          devShells.ci_clang21 = util.mkShellWithCC' pkgs.clang_21;
 
           devShells.ci_zig0_12 = util.mkShellWithCC' (zigWrapCC pkgs.zig_0_12);
           devShells.ci_zig0_13 = util.mkShellWithCC' (zigWrapCC pkgs.zig_0_13);