CMP0170
-------

.. versionadded:: 3.30

When ``FETCHCONTENT_FULLY_DISCONNECTED`` is set to true,
:command:`FetchContent_MakeAvailable` and :command:`FetchContent_Populate`
enforce the constraint that their source directory must already be populated.
The requirement has always been documented, but it was not checked or enforced
with CMake 3.29 or older.  This sometimes led to hard-to-trace errors when a
project expected a dependency to have been populated, but its population was
silently skipped.

CMake 3.30 and above prefers to check and enforce the constraint.
This policy provides compatibility for situations where the user cannot easily
prevent ``FETCHCONTENT_FULLY_DISCONNECTED`` from being inappropriately set
to true.

The ``OLD`` behavior of this policy allows ``FETCHCONTENT_FULLY_DISCONNECTED``
to be set to true even if a dependency's source directory has not been
populated.
The ``NEW`` behavior halts with a fatal error if
``FETCHCONTENT_FULLY_DISCONNECTED`` is set to true and a dependency population
would be skipped, but that dependency's source directory doesn't exist.

.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 3.30
.. |WARNS_OR_DOES_NOT_WARN| replace:: warns
.. include:: include/STANDARD_ADVICE.rst

.. include:: include/DEPRECATED.rst
