Merge pull request #2790 from nlohmann/macos_standards

Add C++ standards to macOS matrix

.github/workflows/macos.yml

@@ -24,3 +24,20 @@ jobs:
         run: cmake --build build --parallel 10
       - name: test
         run: cd build ; ctest -j 10 --output-on-failure
+
+  xcode_standards:
+    runs-on: macos-10.15
+    strategy:
+      matrix:
+        standard: [11, 14, 17, 20]
+    env:
+      DEVELOPER_DIR: /Applications/Xcode_12.4.app/Contents/Developer
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: cmake
+        run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=${{ matrix.standard }} -DCMAKE_CXX_STANDARD_REQUIRED=ON
+      - name: build
+        run: cmake --build build --parallel 10
+      - name: test
+        run: cd build ; ctest -j 10 --output-on-failure