XLClang: Add policy CMP0089 to present as XL for compatibility

We now identify IBM's Clang-based XL compilers, which define
`__ibmxl__`, as `XLClang` rather than `XL`.  In order to support
existing project code that checks for `XL`, add a policy whose OLD
behavior is to present the compiler id as `XL` and whose NEW behavior is
to present the compiler id as `XLClang` as we really detect it.

4 files changed, 45 insertions, 0 deletions

diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 409b5b1..9470d6c 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 3.15
+=================================
+
+.. toctree::
+   :maxdepth: 1
+
+   CMP0089: Compiler id for IBM Clang-based XL compilers is now XLClang. </policy/CMP0089>
+
 Policies Introduced by CMake 3.14
 =================================
 
diff --git a/Help/policy/CMP0089.rst b/Help/policy/CMP0089.rst
new file mode 100644
index 0000000..e549e0c
--- /dev/null
+++ b/Help/policy/CMP0089.rst
@@ -0,0 +1,30 @@
+CMP0089
+-------
+
+Compiler id for IBM Clang-based XL compilers is now ``XLClang``.
+
+CMake 3.15 and above recognize that IBM's Clang-based XL compilers
+that define ``__ibmxl__`` are a new front-end distinct from ``xlc``
+with a different command line and set of capabilities.
+CMake now prefers to present this to projects by setting the
+:variable:`CMAKE_<LANG>_COMPILER_ID` variable to ``XLClang`` instead
+of ``XL``.  However, existing projects may assume the compiler id for
+Clang-based XL is just ``XL`` as it was in CMake versions prior to 3.15.
+Therefore this policy determines for Clang-based XL compilers which
+compiler id to report in the :variable:`CMAKE_<LANG>_COMPILER_ID`
+variable after language ``<LANG>`` is enabled by the :command:`project`
+or :command:`enable_language` command.  The policy must be set prior
+to the invocation of either command.
+
+The OLD behavior for this policy is to use compiler id ``XL``.  The
+NEW behavior for this policy is to use compiler id ``XLClang``.
+
+This policy was introduced in CMake version 3.15.  Use the
+:command:`cmake_policy` command to set this policy to OLD or NEW explicitly.
+Unlike most policies, CMake version |release| does *not* warn
+by default when this policy is not set and simply uses OLD behavior.
+See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0089 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.
+
+.. include:: DEPRECATED.txt
diff --git a/Help/release/dev/add-xlclang.rst b/Help/release/dev/add-xlclang.rst
new file mode 100644
index 0000000..77ff938
--- /dev/null
+++ b/Help/release/dev/add-xlclang.rst
@@ -0,0 +1,5 @@
+add-xlclang
+-----------
+
+* IBM Clang-based XL compilers that define ``__ibmxl__`` now use the
+  compiler id ``XLClang`` instead of ``XL``.  See policy :policy:`CMP0089`.
diff --git a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
index d179728..fc52e7b 100644
--- a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
+++ b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
@@ -21,6 +21,8 @@ warn by default:
   policy :policy:`CMP0067`.
 * ``CMAKE_POLICY_WARNING_CMP0082`` controls the warning for
   policy :policy:`CMP0082`.
+* ``CMAKE_POLICY_WARNING_CMP0089`` controls the warning for
+  policy :policy:`CMP0089`.
 
 This variable should not be set by a project in CMake code.  Project
 developers running CMake may set this variable in their cache to