add_custom_command: Diagnose MAIN_DEPENDENCY limitation.

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

3 files changed, 28 insertions, 0 deletions

diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 96f39e6..76ca5d4 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -114,3 +114,4 @@ All Policies
    /policy/CMP0054
    /policy/CMP0055
    /policy/CMP0056
+   /policy/CMP0057
diff --git a/Help/policy/CMP0057.rst b/Help/policy/CMP0057.rst
new file mode 100644
index 0000000..5cf0784
--- /dev/null
+++ b/Help/policy/CMP0057.rst
@@ -0,0 +1,21 @@
+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.
diff --git a/Help/release/dev/main_dependency_diagnostic.rst b/Help/release/dev/main_dependency_diagnostic.rst
new file mode 100644
index 0000000..13486ef
--- /dev/null
+++ b/Help/release/dev/main_dependency_diagnostic.rst
@@ -0,0 +1,6 @@
+main_dependency_diagnostic
+--------------------------
+
+* Listing the same input file as a MAIN_DEPENDENCY of a custom command
+  can lead to broken build time behavior.  This is now diagnosed.
+  See policy :policy:`CMP0057`.