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, 7 insertions, 2 deletions

diff --git a/Help/command/export.rst b/Help/command/export.rst
index b255ee8..ffd60e1 100644
--- a/Help/command/export.rst
+++ b/Help/command/export.rst
@@ -62,8 +62,13 @@ registry that this command creates works only in conjunction with a
 package configuration file (``<PackageName>Config.cmake``) that works with the
 build tree. In some cases, for example for packaging and for system
 wide installations, it is not desirable to write the user package
-registry. If the :variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` variable
-is enabled, the ``export(PACKAGE)`` command will do nothing.
+registry.
+
+By default the ``export(PACKAGE)`` command does nothing (see policy
+:policy:`CMP0090`) because populating the user package registry has effects
+outside the source and build trees.  Set the
+:variable:`CMAKE_EXPORT_PACKAGE_REGISTRY` variable to add build directories to
+the CMake user package registry.
 
 .. code-block:: cmake