ci: add FetchContent consumer regression test

.github/workflows/pull-request.yml

@@ -93,3 +93,32 @@ jobs:
           cd docs
           npm install 
           npm run build
+
+  verify_fetchcontent:
+    # Graaf is commonly consumed via FetchContent. This builds a minimal
+    # standalone project that does exactly that, to catch regressions in our
+    # own CMakeLists.txt that only surface when Graaf is a subproject rather
+    # than top-level (see #310/#312).
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Configure consumer project
+        run: cmake -B ${{github.workspace}}/consumer-build -S ${{github.workspace}}/cmake_integration/consumer -DGRAAF_SOURCE_DIR=${{github.workspace}}
+
+      - name: Build consumer project
+        run: cmake --build ${{github.workspace}}/consumer-build
+
+      - name: Run consumer executable
+        run: ${{github.workspace}}/consumer-build/consumer
+
+      - name: Install consumer project
+        run: cmake --install ${{github.workspace}}/consumer-build --prefix ${{github.workspace}}/consumer-install
+
+      - name: Verify no pkgconfig file leaked from the FetchContent dependency
+        run: |
+          if [ -f "${{github.workspace}}/consumer-install/lib/pkgconfig/graaf.pc" ]; then
+            echo "::error::graaf.pc should not be installed when Graaf is consumed via FetchContent"
+            exit 1
+          fi

cmake_integration/README.md

@@ -0,0 +1,18 @@
+# CMake integration check
+
+`consumer/` is a minimal standalone CMake project that consumes Graaf via
+`FetchContent`, the same way a real downstream project would. It exists to
+catch regressions in Graaf's own `CMakeLists.txt` that only surface when
+Graaf is built as a subproject rather than top-level (see #310/#312).
+
+Run it locally from the repo root:
+
+```bash
+cmake -B /tmp/graaf-consumer-build -S cmake_integration/consumer -DGRAAF_SOURCE_DIR="$(pwd)"
+cmake --build /tmp/graaf-consumer-build
+cmake --install /tmp/graaf-consumer-build --prefix /tmp/graaf-consumer-install
+```
+
+This should succeed, and `/tmp/graaf-consumer-install/lib/pkgconfig/graaf.pc`
+should **not** exist (pkgconfig generation is only meant for a top-level
+Graaf build).

cmake_integration/consumer/CMakeLists.txt

@@ -0,0 +1,21 @@
+cmake_minimum_required(VERSION 3.11...3.25)
+
+project(GraafFetchContentConsumer LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+include(FetchContent)
+FetchContent_Declare(
+    graaf
+    SOURCE_DIR ${GRAAF_SOURCE_DIR}
+)
+set(SKIP_EXAMPLES ON CACHE BOOL "" FORCE)
+set(SKIP_TESTS ON CACHE BOOL "" FORCE)
+set(SKIP_BENCHMARKS ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(graaf)
+
+add_executable(consumer main.cpp)
+target_link_libraries(consumer PRIVATE Graaf::Graaf)
+
+install(TARGETS consumer RUNTIME DESTINATION bin)

cmake_integration/consumer/main.cpp

@@ -0,0 +1,10 @@
+#include <graaflib/graph.h>
+
+int main() {
+  graaf::undirected_graph<int, float> graph{};
+  const auto vertex_1{graph.add_vertex(1)};
+  const auto vertex_2{graph.add_vertex(2)};
+  graph.add_edge(vertex_1, vertex_2, 1.0F);
+
+  return graph.edge_count() == 1 ? 0 : 1;
+}