CMake/Help/variable/CMAKE_LANG_EXTENSIONS_DEFAULT.rst

CMAKE_<LANG>_EXTENSIONS_DEFAULT
-------------------------------

.. versionadded:: 3.22

Compiler's default extensions mode. Used as the default for the
:prop_tgt:`<LANG>_EXTENSIONS` target property when
:variable:`CMAKE_<LANG>_EXTENSIONS` is not set (see :policy:`CMP0128`).

This variable is read-only.  Modifying it is undefined behavior.