while: diagnose errors during condition evaluation

Add a policy to diagnose condition errors in a compatible way.

Fixes: #23296

Help/manual/cmake-policies.7.rst

@@ -52,6 +52,14 @@ to determine whether to report an error on use of deprecated macros or
 functions.
 
 
+Policies Introduced by CMake 3.24
+=================================
+
+.. toctree::
+   :maxdepth: 1
+
+   CMP0130: while() diagnoses condition evaluation errors. </policy/CMP0130>
+
 Policies Introduced by CMake 3.23
 =================================
 

Help/policy/CMP0130.rst

@@ -0,0 +1,32 @@
+CMP0130
+-------
+
+.. versionadded:: 3.24
+
+:command:`while` diagnoses condition evaluation errors.
+
+CMake 3.23 and below accidentally tolerated errors encountered while
+evaluating the condition passed to the :command:`while` command
+(but not the :command:`if` command).  For example, the code
+
+.. code-block:: cmake
+
+  set(paren "(")
+  while(${paren})
+  endwhile()
+
+creates an unbalanced parenthesis during condition evaluation.
+
+CMake 3.24 and above prefer to diagnose such errors.  This policy
+provides compatibility for projects that have not been updated to
+fix their condition errors.
+
+The ``OLD`` behavior for this policy is to ignore errors in
+:command:`while` conditions.  The ``NEW`` behavior for this
+policy is to diagnose errors in :command:`while` conditions.
+
+This policy was introduced in CMake version 3.24.  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

Help/release/dev/while-errors.rst

@@ -0,0 +1,5 @@
+while-errors
+------------
+
+* The :command:`while` command now diagnoses errors during condition
+  evaluation.  See policy :policy:`CMP0130`.