export: Disable PACKAGE mode user package registry by default

The user package registry populated by the `export()` command causes
side effects outside the build and source directories.  Such effects
should be opt-in rather than op-out.  Introduce a policy to change
default behavior of `export(PACKAGE)` to do nothing.

1 files changed, 27 insertions, 0 deletions

diff --git a/Help/policy/CMP0090.rst b/Help/policy/CMP0090.rst
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+++ b/Help/policy/CMP0090.rst
@@ -0,0 +1,27 @@
+CMP0090
+-------
+
+:command:`export(PACKAGE)` does not populate package registry by default.
+
+In CMake 3.14 and below the :command:`export(PACKAGE)` command populated the
+user package registry by default and users needed to set the
+:variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` to disable it, e.g. in automated
+build and packaging environments.  Since the user package registry is stored
+outside the build tree, this side effect should not be enabled by default.
+Therefore CMake 3.15 and above prefer that :command:`export(PACKAGE)` does
+nothing unless an explicit :variable:`CMAKE_EXPORT_PACKAGE_REGISTRY` variable
+is set to enable it.  This policy provides compatibility with projects that
+have not been updated.
+
+The ``OLD`` behavior for this policy is for :command:`export(PACKAGE)` command
+to populate the user package registry unless
+:variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` is enabled.
+The ``NEW`` behavior is for :command:`export(PACKAGE)` command to do nothing
+unless the :variable:`CMAKE_EXPORT_PACKAGE_REGISTRY` is enabled.
+
+This policy was introduced in CMake version 3.15.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt