CMAKE_CXX_STANDARD: cmake 3.16 vs 3.24

Usage
1

I am observing a change in behavior in between cmake 3.16.3 and 3.24.2.

Basically the following is working as expected in cmake 3.16.3 :

cmake_minimum_required(VERSION 3.10)
project(p)
set(CMAKE_REQUIRED_FLAGS "-std=c++11")
check_cxx_source_compiles("${my_code}" MY_VAR)

However in cmake 3.24.1, the try_compile fails because -std=c++98 is being added after my -std=c++11. Is there a way to preseve the behavior from 3.16 ?

2

@tambre might have some idea what’s going on with this one.

3

Please provide the resulting zipped build directory of a clean build with --debug-trycompile --trace --trace-expand --trace-redirect=trace.txt.

However in cmake 3.24.1, the try_compile fails because -std=c++98 is being added after my -std=c++11.

I presume this is with CMP0128 set to NEW? If there’s a change with it still being OLD then that’s a bug.

4

Here you go:

bad.tgz (131.3 KB)
good.tgz (116.7 KB)

Full code is:

cmake_minimum_required(VERSION 3.10)
if(POLICY CMP0128)
  cmake_policy(SET CMP0128 OLD)
endif()
project(p VERSION 1.0.0)
set(CMAKE_CXX_EXTENSIONS OFF) # important
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}")
find_package(Atomics REQUIRED)

Use case:

Highway is not compiled with any particular CMAKE_CXX_STANDARD set.

5

@tambre do you need any additional details ? thanks

6

Sorry, I’m simply very busy. Should hopefully have the time for a closer look over the weekend.

7

Would you be able to build CMake from source and bisect 3.16.3 and 3.24.2 for this issue?
I’ve combed through the CMP0128 implementation once again and I suspect this is more likely to have resulted from an earlier refactor.

8

Ok, then I’ll try to find some time then. I did report an issue to track progress, and communicate with upstream. Thanks