Add policy CMP0025 for Apple Clang compiler id compatibility

The parent commit introduced a separate "AppleClang" compiler id for
Apple's Clang distribution.  Add a policy in order to support projects
that expect this compiler's id to be just "Clang".  When the policy is
OLD or not set, map AppleClang back to Clang.  Continue to use the
AppleClang id internally while enabling the language, but set the
CMAKE_<LANG>_COMPILER_ID after project() or enable_language() to the
compatible value for use by project code.

1 files changed, 41 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index 7f2b592..2c3834a 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -616,6 +616,9 @@ cmGlobalGenerator::EnableLanguage(std::vector<std::string>const& languages,
       {
       this->LanguageToOriginalSharedLibFlags[lang] = sharedLibFlags;
       }
+
+    // Translate compiler ids for compatibility.
+    this->CheckCompilerIdCompatibility(mf, lang);
     } // end for each language
 
   // Now load files that can override any settings on the platform or for
@@ -632,6 +635,44 @@ cmGlobalGenerator::EnableLanguage(std::vector<std::string>const& languages,
 }
 
 //----------------------------------------------------------------------------
+void cmGlobalGenerator::CheckCompilerIdCompatibility(cmMakefile* mf,
+                                                     std::string lang)
+{
+  std::string compilerIdVar = "CMAKE_" + lang + "_COMPILER_ID";
+  const char* compilerId = mf->GetDefinition(compilerIdVar.c_str());
+  if(compilerId && strcmp(compilerId, "AppleClang") == 0)
+    {
+    cmPolicies* policies = this->CMakeInstance->GetPolicies();
+    switch(mf->GetPolicyStatus(cmPolicies::CMP0025))
+      {
+      case cmPolicies::WARN:
+        if(!this->CMakeInstance->GetIsInTryCompile())
+          {
+          cmOStringStream w;
+          w << policies->GetPolicyWarning(cmPolicies::CMP0025) << "\n"
+            "Converting " << lang <<
+            " compiler id \"AppleClang\" to \"Clang\" for compatibility."
+            ;
+          mf->IssueMessage(cmake::AUTHOR_WARNING, w.str());
+          }
+      case cmPolicies::OLD:
+        // OLD behavior is to convert AppleClang to Clang.
+        mf->AddDefinition(compilerIdVar.c_str(), "Clang");
+        break;
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS:
+        mf->IssueMessage(
+          cmake::FATAL_ERROR,
+          policies->GetRequiredPolicyError(cmPolicies::CMP0025)
+          );
+      case cmPolicies::NEW:
+        // NEW behavior is to keep AppleClang.
+        break;
+      }
+    }
+}
+
+//----------------------------------------------------------------------------
 const char*
 cmGlobalGenerator::GetLanguageOutputExtension(cmSourceFile const& source)
 {