cmMarkAsAdvancedCommand: ignore variables which don't exist in the cache

Fixes: #18331

1 files changed, 25 insertions, 0 deletions

diff --git a/Help/policy/CMP0102.rst b/Help/policy/CMP0102.rst
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+CMP0102
+-------
+
+The :command:`mark_as_advanced` command no longer creates a cache entry if one
+does not already exist.
+
+In CMake 3.16 and below, if a variable was not defined at all or just defined
+locally, the :command:`mark_as_advanced` command would create a new cache
+entry with an ``UNINITIALIZED`` type and no value. When a :command:`find_path`
+(or other similar ``find_`` command) would next run, it would find this
+undefined cache entry and set it up with an empty string value. This process
+would end up deleting the local variable in the process (due to the way the
+cache works), effectively clearing any stored ``find_`` results that were only
+available in the local scope.
+
+The ``OLD`` behavior for this policy is to create the empty cache definition.
+The ``NEW`` behavior of this policy is to ignore variables which do not
+already exist in the cache.
+
+This policy was introduced in CMake version 3.17.  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