Help: Explain policy CMP0125 in more detail

Help/policy/CMP0125.rst

@@ -4,18 +4,35 @@ CMP0125
 .. versionadded:: 3.21
 
 The :command:`find_file`, :command:`find_path`, :command:`find_library` and
-:command:`find_program` commands handle cache variables in the same way
-regardless of whether they are defined on the command line, with or without a
-type, or using the :command:`set` command.
-
-Starting with CMake 3.21, the :command:`find_file`, :command:`find_path`,
-:command:`find_library`, and :command:`find_program` commands ensure that the
-cache variables will be used in the same way regardless how they were defined
-and the result will be always successful if the searched artifact exists.
-
-The ``OLD`` behavior for this policy is to have the find commands' behaviors
-differ depending on how the cache variable is defined. The ``NEW`` behavior for
-this policy is to have consistent behavior.
+:command:`find_program` commands cache their result in the variable specified
+by their first argument.  Prior to CMake 3.21, if a cache variable of that
+name already existed before the call but the cache variable had no type, any
+non-cache variable of the same name would be discarded and the cache variable
+was always used (see also :policy:`CMP0126` for a different but similar
+behavior).  This contradicts the convention that a non-cache variable should
+take precedence over a cache variable of the same name.  Such a situation can
+arise if a user sets a cache variable on the command line without specifying
+a type, such as ``cmake -DMYVAR=blah ...`` instead of
+``cmake -DMYVAR:FILEPATH=blah``.
+
+Related to the above, if a cache variable of the specified name already exists
+and it *does* have a type, the various ``find_...()`` commands would return
+that value unchanged.  In particular, if it contained a relative path, it
+would not be converted to an absolute path in this situation.
+
+When policy ``CMP0125`` is set to ``OLD`` or is unset, the behavior is as
+described above.  When it is set to ``NEW``, the behavior is as follows:
+
+* If a non-cache variable of the specified name exists when the ``find_...()``
+  command is called, its value will be used regardless of whether a cache
+  variable of the same name already exists or not.  A cache variable will not
+  be created in this case if no such cache variable existed before.
+  If a cache variable of the specified name did already exist, the cache will
+  be updated to match the non-cache variable.
+
+* The various ``find...()`` commands will always provide an absolute path in
+  the result variable, except where a relative path provided by a cache or
+  non-cache variable cannot be resolved to an existing path.
 
 This policy was introduced in CMake version 3.21. Use the
 :command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.