CMP0225
-------

.. versionadded:: 4.5

:command:`ExternalProject_Add` sets :variable:`CMAKE_INSTALL_PREFIX` to
``<INSTALL_DIR>`` in the default configure command.

When no ``CONFIGURE_COMMAND`` is given, :command:`ExternalProject_Add` drives
the external project as a CMake project with a default configure command.
CMake 4.4 and below do not pass :variable:`CMAKE_INSTALL_PREFIX` in that
command, so the external project's install step uses CMake's built-in default
prefix, such as ``/usr/local`` on Unix or ``C:/Program Files`` on Windows,
rather than the ``INSTALL_DIR`` that the module reserves for it inside the
build tree.  Installing to a shared system prefix as a side effect of a build
is surprising and often fails for lack of permission.

CMake 4.5 and above prefer to set :variable:`CMAKE_INSTALL_PREFIX` to the
external project's ``INSTALL_DIR``, so that it installs to the location the
module reserves for it.

The ``OLD`` behavior of this policy does not set
:variable:`CMAKE_INSTALL_PREFIX` in the default configure command.  The
``NEW`` behavior sets it to the external project's ``INSTALL_DIR``.

This policy affects only the default configure command; a custom
``CONFIGURE_COMMAND``, including an empty one, is unaffected.

A :variable:`CMAKE_INSTALL_PREFIX` given by the caller in ``CMAKE_ARGS`` (for
example, through :option:`cmake --install-prefix`) or in ``CMAKE_CACHE_ARGS``
still takes precedence.  A prefix given in ``CMAKE_CACHE_DEFAULT_ARGS`` does
not, matching that option's non-overriding-default semantics.

Setting :variable:`CMAKE_INSTALL_PREFIX` in the external project also has
effects beyond the install location:
:variable:`CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT` is left false, and the
prefix is prepended to :variable:`CMAKE_SYSTEM_PREFIX_PATH`, which can change
``find_*()`` results.  These effects apply even when the install step is
disabled with ``INSTALL_COMMAND ""``.

Adopting the ``NEW`` behavior in an existing build tree reconfigures the
external project and overwrites its cached :variable:`CMAKE_INSTALL_PREFIX`;
returning to the ``OLD`` behavior does not restore the previous cached value.
As with the other directory placeholders, changing ``INSTALL_DIR`` later may
not reconfigure the external project with every generator, so its build tree
may need to be cleaned for the cached prefix to update.

.. |INTRODUCED_IN_CMAKE_VERSION| replace:: 4.5
.. |WARNS_OR_DOES_NOT_WARN| replace:: warns
.. include:: include/STANDARD_ADVICE.rst

.. include:: include/DEPRECATED.rst