test(cmake): add test for installable with C++17

.github/workflows/lint.yml

@@ -74,7 +74,7 @@ jobs:
 
       - name: clang-tidy
         run: |
-          make clang-tidy
+          make clang-tidy CMAKE_CXX_STANDARD=17
 
       - name: addlicense
         run: |

.github/workflows/tests.yml

@@ -74,6 +74,13 @@ jobs:
             python -m pip install -r tests/requirements.txt
           fi
 
+      - name: Test installable with C++17
+        shell: bash
+        run: |
+          CMAKE_CXX_STANDARD=17 python -m pip install -vvv --editable .
+          python -X dev -W 'always' -W 'error' -c 'import optree'
+          python -m pip uninstall -y optree
+
       - name: Install OpTree
         run: |
           python -m pip install -vvv --editable '.[test]'

CMakeLists.txt

@@ -32,8 +32,17 @@ if(NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Release)
 endif()
 
-set(CMAKE_CXX_STANDARD 20)  # for likely/unlikely attributes
+if(NOT DEFINED CMAKE_CXX_STANDARD AND NOT "$ENV{CMAKE_CXX_STANDARD}" STREQUAL "")
+    set(CMAKE_CXX_STANDARD "$ENV{CMAKE_CXX_STANDARD}")
+endif()
+if(NOT CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 20)  # for likely/unlikely attributes
+endif()
+if (CMAKE_CXX_STANDARD VERSION_LESS 17)
+    message(FATAL_ERROR "C++17 or higher is required")
+endif()
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+message(STATUS "Use C++ standard: C++${CMAKE_CXX_STANDARD}")
 
 set(CMAKE_POSITION_INDEPENDENT_CODE ON)  # -fPIC
 set(CMAKE_CXX_VISIBILITY_PRESET hidden)  # -fvisibility=hidden

Makefile

@@ -13,7 +13,8 @@ GOBIN          ?= $(GOPATH)/bin
 PATH           := $(PATH):$(GOBIN)
 PYTHON         ?= $(shell command -v python3 || command -v python)
 PYTESTOPTS     ?=
-OPTREE_CXX_WERROR ?= ON
+CMAKE_CXX_STANDARD ?= 20
+OPTREE_CXX_WERROR  ?= ON
 
 .PHONY: default
 default: install
@@ -27,7 +28,8 @@ install-editable install-e:
 	$(PYTHON) -m pip install --upgrade pip
 	$(PYTHON) -m pip install --upgrade setuptools wheel
 	$(PYTHON) -m pip install --upgrade pybind11 cmake
-	OPTREE_CXX_WERROR="$(OPTREE_CXX_WERROR)" $(PYTHON) -m pip install -vvv --no-build-isolation --editable .
+	OPTREE_CXX_WERROR="$(OPTREE_CXX_WERROR)" CMAKE_CXX_STANDARD="$(CMAKE_CXX_STANDARD)" \
+		$(PYTHON) -m pip install -vvv --no-build-isolation --editable .
 
 .PHONY: uninstall
 uninstall:
@@ -198,6 +200,7 @@ cmake-configure: cmake-install
 	cmake --version
 	cmake -S . -B cmake-build-debug \
 		-DCMAKE_BUILD_TYPE=Debug \
+		-DCMAKE_CXX_STANDARD="$(CMAKE_CXX_STANDARD)" \
 		-DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
 		-DPython_EXECUTABLE="$(PYTHON)" \
 		-DOPTREE_CXX_WERROR="$(OPTREE_CXX_WERROR)"
@@ -225,7 +228,8 @@ clang-tidy: clang-tidy-install cmake-configure
 
 .PHONY: addlicense
 addlicense: addlicense-install
-	addlicense -c $(COPYRIGHT) -ignore tests/coverage.xml -l apache -y 2022-$(shell date +"%Y") -check $(SOURCE_FOLDERS)
+	addlicense -c $(COPYRIGHT) -l apache -y 2022-$(shell date +"%Y") \
+		-ignore tests/coverage.xml -check $(SOURCE_FOLDERS)
 
 .PHONY: docstyle
 docstyle: docs-install
@@ -256,7 +260,8 @@ format: py-format-install ruff-install clang-format-install addlicense-install
 	$(PYTHON) -m black $(PYTHON_FILES)
 	$(PYTHON) -m ruff check --fix --exit-zero .
 	$(CLANG_FORMAT) -style=file -i $(CXX_FILES)
-	addlicense -c $(COPYRIGHT) -ignore tests/coverage.xml -l apache -y 2022-$(shell date +"%Y") $(SOURCE_FOLDERS)
+	addlicense -c $(COPYRIGHT) -l apache -y 2022-$(shell date +"%Y") \
+		-ignore tests/coverage.xml $(SOURCE_FOLDERS)
 
 .PHONY: clean-py
 clean-py: