make sure cabal-install is compatible with Cabal

See #9833

If a ghc ships with a compatible Cabal, it will be preferred by
the solver on `cabal install cabal-install`; the new `cabal-install`
should in fact be compatible. So we test this on release branches
that have had at least one release on Hackage. (Ideally we'd check
ghc instead, but we can't do that from GHA. Even checking Hackage
is pretty painful.)

.github/workflows/check-sdist.yml

@@ -0,0 +1,82 @@
+name: Check sdist
+
+# See: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency.
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: true
+
+on:
+  push:
+    paths-ignore:
+      - "doc/**"
+      - "**/README.md"
+      - "CONTRIBUTING.md"
+    branches:
+      - master
+  pull_request:
+    paths-ignore:
+      - "doc/**"
+      - "**/README.md"
+      - "CONTRIBUTING.md"
+  release:
+    types:
+      - created
+  workflow_call:
+
+jobs:
+
+  # Dogfood the generated sdist, to avoid bugs like https://github.com/haskell/cabal/issues/9833
+  # No caching, since the point is to verify they can be installed "from scratch"
+  # Don't run on master or a PR targeting master, because there's never an installable Cabal
+  dogfood-sdists:
+    name: Dogfood sdist on ${{ matrix.os }} ghc-${{ matrix.ghc }}
+    # if: github.ref != 'refs/heads/master' && github.base_ref != 'master'
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        # this should be kept up to date with the list in validate.yml
+        # sharing these with the main validate job is possible but extremely painful; sadly,
+        # you can't simply reference another job's matrix
+        ghc:
+          [
+            "9.10.1",
+            "9.8.2",
+            "9.6.4",
+            "9.4.8",
+            "9.2.8",
+            "9.0.2",
+            "8.10.7",
+            "8.8.4",
+          ]
+
+    steps:
+
+      - uses: haskell-actions/setup@v2
+        id: setup-haskell
+        with:
+          ghc-version: ${{ matrix.ghc }}
+          cabal-version: latest
+
+      - uses: actions/checkout@v4
+
+      - name: Make sdist
+        run: cabal sdist cabal-install
+
+      - name: Install from sdist
+        run: |
+          # skip if a suitable Cabal isn't in the index (i.e. new major version, not released yet)
+          # we only want to test cabal-install, to ensure that it works with existing Cabals
+          # (don't look at this too closely)
+          sdist="$(ls dist-newstyle/sdist/cabal-install-*.tar.gz | sed -n '\,^dist-newstyle/sdist/cabal-install-[0-9.]*\.tar\.gz$,{;p;q;}')"
+          # extract the cabal-install major version
+          ver="$(echo "$sdist" | sed -n 's,^dist-newstyle/sdist/cabal-install-\([0-9][0-9]*\.[0-9][0-9]*\)\.[0-9.]*$,\1,p')"
+          # why does `cabal list` force me to do this???
+          #@@@ temporarily always run to ensure this doesn't pull Cabal from the tree
+          #if cabal list --simple-output Cabal | grep -q "^Cabal $cbl\\."; then
+            # I sure hope this works…
+            cabal build "$sdist" --prefer-oldest --ignore-project
+          #else
+          #  echo No released Cabal version to test against.
+          #  exit 0
+          #fi