Added in version 3.22.
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.
This policy was introduced in CMake version 3.22.
It may be set by cmake_policy() or cmake_minimum_required().
If it is not set, CMake does not warn by default, and uses OLD behavior.
See documentation of the
CMAKE_POLICY_WARNING_CMP0128
variable to control the warning.
Note
The OLD behavior of a policy is
deprecated by definition
and may be removed in a future version of CMake.