haskellPackages.ghcWithPackages: throw on old override interface

Adding a fake override function via passthru will at least give users of
the old override interface a more helpful error message. Additionally we
also document the changes in the changelog.

nixos/doc/manual/from_md/release-notes/rl-2205.section.xml

@@ -240,6 +240,19 @@
           <literal>haskellPackages.callPackage</literal>).
         </para>
       </listitem>
+      <listitem>
+        <para>
+          <literal>pkgs.ghc.withPackages</literal> as well as
+          <literal>haskellPackages.ghcWithPackages</literal> etc. now
+          needs be overridden directly, as opposed to overriding the
+          result of calling it. Additionally, the
+          <literal>withLLVM</literal> parameter has been renamed to
+          <literal>useLLVM</literal>. So instead of
+          <literal>(ghc.withPackages (p: [])).override { withLLVM = true; }</literal>,
+          one needs to use
+          <literal>(ghc.withPackages.override { useLLVM = true; }) (p: [])</literal>.
+        </para>
+      </listitem>
       <listitem>
         <para>
           <literal>pkgs.emacsPackages.orgPackages</literal> is removed

nixos/doc/manual/release-notes/rl-2205.section.md

@@ -81,6 +81,12 @@ In addition to numerous new and upgraded packages, this release has the followin
   instead to ensure cross compilation keeps working (or switch to
   `haskellPackages.callPackage`).
 
+- `pkgs.ghc.withPackages` as well as `haskellPackages.ghcWithPackages` etc.
+  now needs be overridden directly, as opposed to overriding the result of
+  calling it. Additionally, the `withLLVM` parameter has been renamed to
+  `useLLVM`. So instead of `(ghc.withPackages (p: [])).override { withLLVM = true; }`,
+  one needs to use `(ghc.withPackages.override { useLLVM = true; }) (p: [])`.
+
 - `pkgs.emacsPackages.orgPackages` is removed because org elpa is deprecated.
   The packages in the top level of `pkgs.emacsPackages`, such as org and
   org-contrib, refer to the ones in `pkgs.emacsPackages.elpaPackages` and

pkgs/development/haskell-modules/with-packages-wrapper.nix

@@ -164,5 +164,20 @@ symlinkJoin {
   passthru = {
     preferLocalBuild = true;
     inherit (ghc) version meta;
+
+    # Inform users about backwards incompatibilities with <= 21.05
+    override = _: throw ''
+      The ghc.withPackages wrapper itself can now be overridden, but no longer
+      the result of calling it (as before). Consequently overrides need to be
+      adjusted: Instead of
+
+        (ghc.withPackages (p: [ p.my-package ])).override { withLLLVM = true; }
+
+      use
+
+        (ghc.withPackages.override { useLLVM = true; }) (p: [ p.my-package ])
+
+      Also note that withLLVM has been renamed to useLLVM for consistency with
+      the GHC Nix expressions.'';
   };
 }