Features: Define meaning for no language standard default

Define an empty string in CMAKE_<LANG>_STANDARD_DEFAULT to mean that
the toolchain has no notion of lanuage standard levels.  In this case
the <LANG>_STANDARD[_REQUIRED] properties will have no effect.

Update the RunCMake.CompileFeatures test to exclude the
LinkImplementationFeatureCycle test when there is no standard default.
It can never fail because no use of specific features will adjust the
CXX_STANDARD level required for any target since the standard levels
have no meaning in this case.

1 files changed, 7 insertions, 2 deletions

diff --git a/Source/cmLocalGenerator.cxx b/Source/cmLocalGenerator.cxx
index 834f705..af4c950 100644
--- a/Source/cmLocalGenerator.cxx
+++ b/Source/cmLocalGenerator.cxx
@@ -2207,6 +2207,13 @@ AddCompilerRequirementFlag(std::string &flags, cmTarget* target,
     {
     return;
     }
+  const char* defaultStd
+      = this->Makefile->GetDefinition("CMAKE_" + lang + "_STANDARD_DEFAULT");
+  if (defaultStd && !*defaultStd)
+    {
+    // This compiler has no notion of language standard levels.
+    return;
+    }
   std::string stdProp = lang + "_STANDARD";
   const char *standardProp = target->GetProperty(stdProp);
   if (!standardProp)
@@ -2269,8 +2276,6 @@ AddCompilerRequirementFlag(std::string &flags, cmTarget* target,
                                 std::find(stds.begin(), stds.end(), standard);
   assert(stdIt != stds.end());
 
-  const char* defaultStd
-      = this->Makefile->GetDefinition("CMAKE_" + lang + "_STANDARD_DEFAULT");
   std::vector<std::string>::const_iterator defaultStdIt;
   if (defaultStd)
     {