CMP0156
-------

.. versionadded:: 3.29

De-duplicate libraries on link lines based on linker capabilities.

Traditional linkers maintain a set of undefined symbols during linking. The
linker processes each file in the order in which it appears on the command
line, until the set of undefined symbols becomes empty. An object file is
linked into the output object when it is encountered, with its undefined
symbols added to the set. Upon encountering an archive file a traditional
linker searches the objects contained therein, and processes those that satisfy
symbols in the unresolved set.

Handling mutually dependent archives may be awkward when using a traditional
linker. Archive files may have to be specified multiple times.

Some linkers (for instance Apple or Windows linkers, as well as``LLVM LLD``)
records all symbols found in objects and archives as it iterates over command
line arguments. When one of these linkers encounters an undefined symbol that
can be resolved by an object file contained in a previously processed archive
file, it immediately extracts and links it into the output object.

CMake 3.28 and below may generate link lines that repeat static libraries as
a traditional linker would need, even when using a linker does not need it.
They may also de-duplicate shared libraries by keeping their last occurrence,
which on Windows platforms can change DLL load order.

CMake 3.29 and above prefer to apply different strategies based on linker
capabilities.  So, when targeting Apple and Windows platforms, all
libraries are de-duplicated.  Moreover, on Windows platforms, libraries
are de-duplicated by keeping their fist occurrence, thus respecting the
project-specified order.  This policy provides compatibility with projects
that have not been updated to expect the latter behavior.

The ``OLD`` behavior for this policy is to always repeat static libraries
as if using a traditional linker, and always de-duplicate shared libraries
by keeping the last occurrence of each.  The ``NEW`` behavior for this policy
is to apply different strategies based on linker capabilities.

This policy was introduced in CMake version 3.29.  Use the
:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
Unlike many policies, CMake version |release| does *not* warn
when this policy is not set and simply uses ``OLD`` behavior.

.. include:: DEPRECATED.txt
