Genex: Add policy to handle empty list items in $<IN_LIST:...>

The old behavior of $<IN_LIST:...> is inconsistent with that of
if(IN_LIST), in that it does not find an empty search item even if
the list contains empty items. This change adds a new policy to
correctly handle empty items and make the behavior more consistent
with if(IN_LIST).

Fixes: #18556

1 files changed, 21 insertions, 0 deletions

diff --git a/Help/policy/CMP0085.rst b/Help/policy/CMP0085.rst
new file mode 100644
index 0000000000..d9ec9a2079
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+++ b/Help/policy/CMP0085.rst
@@ -0,0 +1,21 @@
+CMP0085
+-------
+
+``$<IN_LIST:...>`` handles empty list items.
+
+In CMake 3.13 and lower, the ``$<IN_LIST:...>`` generator expression always
+returned ``0`` if the first argument was empty, even if the list contained an
+empty item. This behavior is inconsistent with the ``IN_LIST`` behavior of
+:command:`if`, which this generator expression is meant to emulate. CMake 3.14
+and later handles this case correctly.
+
+The ``OLD`` behavior of this policy is for ``$<IN_LIST:...>`` to always return
+``0`` if the first argument is empty. The ``NEW`` behavior is to return ``1``
+if the first argument is empty and the list contains an empty item.
+
+This policy was introduced in CMake version 3.14.  CMake version
+|release| warns when the policy is not set and uses ``OLD`` behavior.
+Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW``
+explicitly.
+
+.. include:: DEPRECATED.txt