add_custom_command: Diagnose MAIN_DEPENDENCY limitation.

The new policy CMP0057 diagnoses reuse of the same MAIN_DEPENDENCY across
multiple custom commands.

1 files changed, 21 insertions, 0 deletions

diff --git a/Help/policy/CMP0057.rst b/Help/policy/CMP0057.rst
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+CMP0057
+-------
+
+Disallow multiple ``MAIN_DEPENDENCY`` specifications for the same file.
+
+CMake 3.3 and above no longer allow the same input file to be used
+as a ``MAIN_DEPENDENCY`` in more than one custom command.
+
+Listing the same input file more than once in this context has not been
+supported by earlier versions either and would lead to build time issues
+but was not diagnosed.
+
+The ``OLD`` behavior for this policy is to allow using the same input file
+in a ``MAIN_DEPENDENCY`` specfication more than once.
+The ``NEW`` behavior is to disallow using the same input file in a
+``MAIN_DEPENDENCY`` specification more than once.
+
+This policy was introduced in CMake version 3.3.
+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.