fix: C++17 mode on Clang may error

tests/CMakeLists.txt

@@ -152,10 +152,10 @@ function(pybind11_enable_warnings target_name)
     endif()
   endif()
 
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
-    if(CMAKE_CXX_STANDARD STREQUAL "14")
+  if(CMAKE_CXX_STANDARD AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+    if(CMAKE_CXX_STANDARD LESS 17)
       target_compile_options(${target_name} PUBLIC -Wno-deprecated-register)
-    elseif(NOT CMAKE_CXX_STANDARD VERSION_LESS 17)
+    else()
       target_compile_options(${target_name} PUBLIC -Wno-register)
     endif()
   endif()

tests/test_cmake_build/installed_embed/CMakeLists.txt

@@ -8,8 +8,17 @@ message(STATUS "Found pybind11 v${pybind11_VERSION}: ${pybind11_INCLUDE_DIRS}")
 add_executable(test_cmake_build ../embed.cpp)
 target_link_libraries(test_cmake_build PRIVATE pybind11::embed)
 
+
 # Do not treat includes from IMPORTED target as SYSTEM (Python headers in pybind11::embed).
 # This may be needed to resolve header conflicts, e.g. between Python release and debug headers.
 set_target_properties(test_cmake_build PROPERTIES NO_SYSTEM_FROM_IMPORTED ON)
 
 add_custom_target(check $<TARGET_FILE:test_cmake_build> ${PROJECT_SOURCE_DIR}/../test.py)
+
+if(CMAKE_CXX_STANDARD AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+  if(CMAKE_CXX_STANDARD LESS 17)
+    target_compile_options(test_cmake_build PUBLIC -Wno-deprecated-register)
+  else()
+    target_compile_options(test_cmake_build PUBLIC -Wno-register)
+  endif()
+endif()

tests/test_cmake_build/installed_target/CMakeLists.txt

@@ -20,3 +20,11 @@ set_target_properties(test_cmake_build PROPERTIES NO_SYSTEM_FROM_IMPORTED ON)
 
 add_custom_target(check ${CMAKE_COMMAND} -E env PYTHONPATH=$<TARGET_FILE_DIR:test_cmake_build>
                   ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/../test.py ${PROJECT_NAME})
+
+if(CMAKE_CXX_STANDARD AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+  if(CMAKE_CXX_STANDARD LESS 17)
+    target_compile_options(test_cmake_build PUBLIC -Wno-deprecated-register)
+  else()
+    target_compile_options(test_cmake_build PUBLIC -Wno-register)
+  endif()
+endif()

tests/test_cmake_build/subdirectory_embed/CMakeLists.txt

@@ -23,3 +23,13 @@ install(TARGETS test_embed_lib
         RUNTIME DESTINATION lib)
 install(EXPORT      test_export
         DESTINATION lib/cmake/test_export/test_export-Targets.cmake)
+
+if(CMAKE_CXX_STANDARD AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+  if(CMAKE_CXX_STANDARD LESS 17)
+    target_compile_options(test_embed_lib PUBLIC -Wno-deprecated-register)
+    target_compile_options(test_cmake_build PUBLIC -Wno-deprecated-register)
+  else()
+    target_compile_options(test_embed_lib PUBLIC -Wno-register)
+    target_compile_options(test_cmake_build PUBLIC -Wno-register)
+  endif()
+endif()

tests/test_cmake_build/subdirectory_target/CMakeLists.txt

@@ -13,3 +13,11 @@ set_target_properties(test_cmake_build PROPERTIES PREFIX "${PYTHON_MODULE_PREFIX
 
 add_custom_target(check ${CMAKE_COMMAND} -E env PYTHONPATH=$<TARGET_FILE_DIR:test_cmake_build>
                   ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/../test.py ${PROJECT_NAME})
+
+if(CMAKE_CXX_STANDARD AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+  if(CMAKE_CXX_STANDARD LESS 17)
+    target_compile_options(test_cmake_build PUBLIC -Wno-deprecated-register)
+  else()
+    target_compile_options(test_cmake_build PUBLIC -Wno-register)
+  endif()
+endif()

tests/test_embed/CMakeLists.txt

@@ -43,3 +43,13 @@ endforeach()
 add_dependencies(cpptest external_module)
 
 add_dependencies(check cpptest)
+
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND PYTHON_VERSION VERSION_LESS 3.0)
+  if(CMAKE_CXX_STANDARD LESS 17)
+    target_compile_options(test_embed PUBLIC -Wno-deprecated-register)
+    target_compile_options(external_module PUBLIC -Wno-deprecated-register)
+  else()
+    target_compile_options(test_embed PUBLIC -Wno-register)
+    target_compile_options(external_module PUBLIC -Wno-register)
+  endif()
+endif()

tools/pybind11Tools.cmake

@@ -224,6 +224,14 @@ function(pybind11_add_module target_name)
     endif()
   endif()
 
+  # Python 2 doesn't really support C++17, Clang warns/errors over it
+  if(CMAKE_CXX_STANDARD
+     AND CMAKE_CXX_COMPILER_ID MATCHES "Clang"
+     AND PYTHON_VERSION VERSION_LESS 3.0
+     AND NOT CMAKE_CXX_STANDARD LESS 17)
+       target_compile_options(${target_name} PUBLIC -Wno-register)
+  endif()
+
   if(ARG_NO_EXTRAS)
     return()
   endif()