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.

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
 =================================
 

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

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`.

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

Source/cmGlobalGenerator.cxx

@@ -956,6 +956,36 @@ void cmGlobalGenerator::CheckCompilerIdCompatibility(
         break;
     }
   }
+
+  if (strcmp(compilerId, "XLClang") == 0) {
+    switch (mf->GetPolicyStatus(cmPolicies::CMP0089)) {
+      case cmPolicies::WARN:
+        if (!this->CMakeInstance->GetIsInTryCompile() &&
+            mf->PolicyOptionalWarningEnabled("CMAKE_POLICY_WARNING_CMP0089")) {
+          std::ostringstream w;
+          /* clang-format off */
+          w << cmPolicies::GetPolicyWarning(cmPolicies::CMP0089) << "\n"
+            "Converting " << lang <<
+            " compiler id \"XLClang\" to \"XL\" for compatibility."
+            ;
+          /* clang-format on */
+          mf->IssueMessage(MessageType::AUTHOR_WARNING, w.str());
+        }
+        CM_FALLTHROUGH;
+      case cmPolicies::OLD:
+        // OLD behavior is to convert XLClang to XL.
+        mf->AddDefinition(compilerIdVar, "XL");
+        break;
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS:
+        mf->IssueMessage(
+          MessageType::FATAL_ERROR,
+          cmPolicies::GetRequiredPolicyError(cmPolicies::CMP0089));
+      case cmPolicies::NEW:
+        // NEW behavior is to keep AppleClang.
+        break;
+    }
+  }
 }
 
 std::string cmGlobalGenerator::GetLanguageOutputExtension(

Source/cmPolicies.h

@@ -261,7 +261,10 @@ class cmMakefile;
          3, 14, 0, cmPolicies::WARN)                                          \
   SELECT(POLICY, CMP0088,                                                     \
          "FindBISON runs bison in CMAKE_CURRENT_BINARY_DIR when executing.",  \
-         3, 14, 0, cmPolicies::WARN)
+         3, 14, 0, cmPolicies::WARN)                                          \
+  SELECT(POLICY, CMP0089,                                                     \
+         "Compiler id for IBM Clang-based XL compilers is now XLClang.", 3,   \
+         15, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \

Tests/RunCMake/ParseImplicitIncludeInfo/data/CMakeLists.txt

@@ -12,6 +12,9 @@
 #
 
 cmake_minimum_required(VERSION 3.3)
+if(POLICY CMP0089)
+  cmake_policy(SET CMP0089 NEW)
+endif()
 
 set(lngs C CXX)
 set(LANGUAGES "${lngs}" CACHE STRING "List of languages to generate inputs for")