export(): raise an error on multiple calls with same FILE

Fixes: 20472

3 files changed, 35 insertions, 0 deletions

diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index c256250..08d59e7 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -51,6 +51,14 @@ The :variable:`CMAKE_MINIMUM_REQUIRED_VERSION` variable may also be used
 to determine whether to report an error on use of deprecated macros or
 functions.
 
+Policies Introduced by CMake 3.18
+=================================
+
+.. toctree::
+   :maxdepth: 1
+
+   CMP0103: Multiple export() with same FILE without APPEND is not allowed. </policy/CMP0103>
+
 Policies Introduced by CMake 3.17
 =================================
 
diff --git a/Help/policy/CMP0103.rst b/Help/policy/CMP0103.rst
new file mode 100644
index 0000000..223e0cb
--- /dev/null
+++ b/Help/policy/CMP0103.rst
@@ -0,0 +1,22 @@
+CMP0103
+-------
+
+Multiple calls to :command:`export` command with same ``FILE`` without
+``APPEND`` is no longer allowed.
+
+In CMake 3.17 and below, multiple calls to :command:`export` command with the
+same ``FILE`` without ``APPEND`` are accepted silently but only the last
+occurrence is taken into account during the generation.
+
+The ``OLD`` behavior for this policy is to ignore the multiple occurrences of
+ :command:`export` command except the last one.
+
+The ``NEW`` behavior of this policy is to raise an error on second call to
+:command:`export` command with same ``FILE`` without ``APPEND``.
+
+This policy was introduced in CMake version 3.18.  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
diff --git a/Help/release/dev/export-multiple-calls.rst b/Help/release/dev/export-multiple-calls.rst
new file mode 100644
index 0000000..00372ce
--- /dev/null
+++ b/Help/release/dev/export-multiple-calls.rst
@@ -0,0 +1,5 @@
+export-multiple-calls
+---------------------
+
+* The :command:`export` command now raise an error if used multiple times with
+  same ``FILE`` without ``APPEND``. See policy :policy:`CMP0103`.