CMP0128

When this policy is set to NEW:

• <LANG>_EXTENSIONS is initialized to CMAKE_<LANG>_EXTENSIONS if set, otherwise falling back to CMAKE_<LANG>_EXTENSIONS_DEFAULT.
• Extensions are correctly enabled/disabled if <LANG>_STANDARD is unset or satisfied by the default.
• Standard mode-affecting flags aren't added unless necessary to achieve the specified mode.

The OLD behavior:

• Initializes <LANG>_EXTENSIONS to CMAKE_<LANG>_EXTENSIONS if set, otherwise falling back to ON.
• Always adds a flag if <LANG>_STANDARD is set and <LANG>_STANDARD_REQUIRED is OFF.

• If <LANG>_STANDARD is unset:

  • Doesn't disable extensions even if <LANG>_EXTENSIONS is OFF.
  • Fails to enable extensions if <LANG>_EXTENSIONS is ON except for the IAR compiler.

Code may need to be updated for the NEW behavior in the following cases:

• If a standard mode flag previously overridden by CMake's and not used during compiler detection now takes effect due to CMake no longer adding one as the default detected is appropriate.

  Such code should be converted to either:

  • Use <LANG>_STANDARD and <LANG>_EXTENSIONS instead of manually adding flags.
  • Or ensure the manually-specified flags are used during compiler detection.

• If extensions were disabled without <LANG>_STANDARD being set CMake previously wouldn't actually disable extensions.

  Such code should be updated to not disable extensions if they are required.

• If extensions were enabled/disabled when <LANG>_STANDARD was satisfied by the compiler's default CMake previously wouldn't actually enable/disable extensions.

  Such code should be updated to set the correct extensions mode.

If compiler flags affecting the standard mode are used during compiler detection (for example in a toolchain file using CMAKE_<LANG>_FLAGS_INIT) then they will affect the detected default standard and extensions.

Unlike many policies, CMake version 3.23.3 does not warn when the policy is not set and simply uses the OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly. See documentation of the CMAKE_POLICY_WARNING_CMP0128 variable to control the warning.