diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 75abc440a..ba36f8995 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -108,3 +108,160 @@ jobs:
${{ runner.temp }}/${{ env.BASELINE_FILE }}
${{ runner.temp }}/${{ env.COMPARE_FILE }}
${{ runner.temp }}/${{ env.COMPARE_OUT_FILE }}
+ crossrepo:
+ name: Cross-Repo Validation
+ runs-on: ubuntu-latest
+ container:
+ image: swift:6.3-jammy
+ steps:
+ - name: Checkout repo
+ uses: actions/checkout@v7
+ - name: Install apt dependencies
+ run: apt-get -qq update && apt-get -qq -y install curl ninja-build
+ - name: Install CMake 3.30.2
+ run: |
+ ARCH=`uname -m`
+ curl -sL https://github.com/Kitware/CMake/releases/download/v3.30.2/cmake-3.30.2-linux-$ARCH.tar.gz -O
+ tar xzf cmake-3.30.2-linux-$ARCH.tar.gz
+ export PATH="`pwd`/cmake-3.30.2-linux-$ARCH/bin:$PATH"
+ echo "PATH=$PATH" >> $GITHUB_ENV
+ cmake --version
+ - name: Compare CMakeLists.txt against swiftlang/swift/Runtimes/Supplemental
+ id: compare
+ env:
+ UPSTREAM_DIR: https://raw.githubusercontent.com/swiftlang/swift/refs/heads/main/Runtimes/Supplemental/StringProcessing
+ MODULES: _StringProcessing _RegexParser
+ shell: bash
+ run: |
+ set -u
+
+ # `-f` so a file moving upstream reports a fetch error rather than
+ # diffing against a 404 page.
+ for module in $MODULES; do
+ if ! curl -fsSL "$UPSTREAM_DIR/$module/CMakeLists.txt" \
+ -o "upstream-$module-CMakeLists.txt"; then
+ echo "result=fetch-failed" >> "$GITHUB_OUTPUT"
+ echo "Could not fetch $module/CMakeLists.txt from swiftlang/swift."
+ exit 1
+ fi
+ done
+
+ : > "$RUNNER_TEMP/cmake-diff.txt"
+ status=0
+ for module in $MODULES; do
+ if ! diff -u \
+ --label "swiftlang/swift: Runtimes/Supplemental/StringProcessing/$module/CMakeLists.txt" \
+ --label "this repo: Sources/$module/CMakeLists.txt" \
+ "upstream-$module-CMakeLists.txt" \
+ "Sources/$module/CMakeLists.txt" \
+ >> "$RUNNER_TEMP/cmake-diff.txt"; then
+ status=1
+ fi
+ done
+
+ if [ "$status" -eq 0 ]; then
+ echo "result=match" >> "$GITHUB_OUTPUT"
+ echo "Both CMakeLists.txt files match swiftlang/swift."
+ else
+ echo "result=differs" >> "$GITHUB_OUTPUT"
+ cat "$RUNNER_TEMP/cmake-diff.txt"
+ fi
+ exit "$status"
+ - name: Configure CMake
+ id: configure
+ if: ${{ !cancelled() }}
+ run: cmake -G Ninja -S Sources -B build -DCMAKE_BUILD_TYPE=Release
+ - name: Build
+ id: build
+ if: ${{ !cancelled() }}
+ run: cmake --build build
+ - name: Verdict
+ if: ${{ always() }}
+ env:
+ COMPARE_OUTCOME: ${{ steps.compare.outcome }}
+ COMPARE_RESULT: ${{ steps.compare.outputs.result }}
+ CONFIGURE_OUTCOME: ${{ steps.configure.outcome }}
+ BUILD_OUTCOME: ${{ steps.build.outcome }}
+ shell: bash
+ run: |
+ set -u
+
+ # Report to the step log and to the job summary page.
+ emit() { printf '%s\n' "$@" | tee -a "$GITHUB_STEP_SUMMARY"; }
+
+ failed=0
+
+ # --- the CMake build ---
+ if [ "$CONFIGURE_OUTCOME" = failure ] || [ "$BUILD_OUTCOME" = failure ]; then
+ failed=1
+ if [ "$CONFIGURE_OUTCOME" = failure ]; then
+ stage='CMake configuration'
+ else
+ stage='CMake build'
+ fi
+ emit \
+ "## :x: $stage failed" \
+ '' \
+ 'The CMake build of `_RegexParser` and `_StringProcessing` is broken.' \
+ "See the \"$stage\" step above for the output." \
+ ''
+ fi
+
+ # --- the comparison against swiftlang/swift ---
+ case "${COMPARE_RESULT:-}" in
+ match)
+ ;;
+ differs)
+ failed=1
+ emit \
+ '## :warning: `CMakeLists.txt` differs from swiftlang/swift' \
+ '' \
+ 'Diff to apply upstream
' \
+ '' \
+ '```diff'
+ tee -a "$GITHUB_STEP_SUMMARY" < "$RUNNER_TEMP/cmake-diff.txt"
+ emit '```' '' '