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.

3 files changed, 60 insertions, 6 deletions

diff --git a/Source/cmGeneratorExpressionEvaluator.cxx b/Source/cmGeneratorExpressionEvaluator.cxx
index 5f246f9..6692a92 100644
--- a/Source/cmGeneratorExpressionEvaluator.cxx
+++ b/Source/cmGeneratorExpressionEvaluator.cxx
@@ -1370,6 +1370,8 @@ static const struct CompileFeaturesNode : public cmGeneratorExpressionNode
       {
       std::vector<std::string> const& langAvailable
                                               = availableFeatures[lit->first];
+      const char* standardDefault = context->Makefile
+        ->GetDefinition("CMAKE_" + lit->first + "_STANDARD_DEFAULT");
       for (std::vector<std::string>::const_iterator it = lit->second.begin();
           it != lit->second.end(); ++it)
         {
@@ -1378,6 +1380,12 @@ static const struct CompileFeaturesNode : public cmGeneratorExpressionNode
           {
           return "0";
           }
+        if (standardDefault && !*standardDefault)
+          {
+          // This compiler has no notion of language standard levels.
+          // All features known for the language are always available.
+          continue;
+          }
         if (!context->Makefile->HaveStandardAvailable(target,
                                                       lit->first, *it))
           {
@@ -1386,8 +1394,7 @@ static const struct CompileFeaturesNode : public cmGeneratorExpressionNode
             const char* l = target->GetProperty(lit->first + "_STANDARD");
             if (!l)
               {
-              l = context->Makefile
-                  ->GetDefinition("CMAKE_" + lit->first + "_STANDARD_DEFAULT");
+              l = standardDefault;
               }
             assert(l);
             context->MaxLanguageStandard[target][lit->first] = l;
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)
     {
diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index 87e62d7..ba914e1 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -5253,6 +5253,27 @@ bool cmMakefile::
 HaveCStandardAvailable(cmTarget const* target,
                        const std::string& feature) const
 {
+  const char* defaultCStandard =
+    this->GetDefinition("CMAKE_C_STANDARD_DEFAULT");
+  if (!defaultCStandard)
+    {
+    std::ostringstream e;
+    e << "CMAKE_C_STANDARD_DEFAULT is not set.  COMPILE_FEATURES support "
+      "not fully configured for this compiler.";
+    this->IssueMessage(cmake::INTERNAL_ERROR, e.str());
+    // Return true so the caller does not try to lookup the default standard.
+    return true;
+    }
+  if (std::find_if(cmArrayBegin(C_STANDARDS), cmArrayEnd(C_STANDARDS),
+                cmStrCmp(defaultCStandard)) == cmArrayEnd(C_STANDARDS))
+    {
+    std::ostringstream e;
+    e << "The CMAKE_C_STANDARD_DEFAULT variable contains an "
+         "invalid value: \"" << defaultCStandard << "\".";
+    this->IssueMessage(cmake::INTERNAL_ERROR, e.str());
+    return false;
+    }
+
   bool needC90 = false;
   bool needC99 = false;
   bool needC11 = false;
@@ -5262,7 +5283,7 @@ HaveCStandardAvailable(cmTarget const* target,
   const char *existingCStandard = target->GetProperty("C_STANDARD");
   if (!existingCStandard)
     {
-    existingCStandard = this->GetDefinition("CMAKE_C_STANDARD_DEFAULT");
+    existingCStandard = defaultCStandard;
     }
 
   if (std::find_if(cmArrayBegin(C_STANDARDS), cmArrayEnd(C_STANDARDS),
@@ -5331,6 +5352,27 @@ bool cmMakefile::IsLaterStandard(std::string const& lang,
 bool cmMakefile::HaveCxxStandardAvailable(cmTarget const* target,
                                          const std::string& feature) const
 {
+  const char* defaultCxxStandard =
+    this->GetDefinition("CMAKE_CXX_STANDARD_DEFAULT");
+  if (!defaultCxxStandard)
+    {
+    std::ostringstream e;
+    e << "CMAKE_CXX_STANDARD_DEFAULT is not set.  COMPILE_FEATURES support "
+      "not fully configured for this compiler.";
+    this->IssueMessage(cmake::INTERNAL_ERROR, e.str());
+    // Return true so the caller does not try to lookup the default standard.
+    return true;
+    }
+  if (std::find_if(cmArrayBegin(CXX_STANDARDS), cmArrayEnd(CXX_STANDARDS),
+                cmStrCmp(defaultCxxStandard)) == cmArrayEnd(CXX_STANDARDS))
+    {
+    std::ostringstream e;
+    e << "The CMAKE_CXX_STANDARD_DEFAULT variable contains an "
+         "invalid value: \"" << defaultCxxStandard << "\".";
+    this->IssueMessage(cmake::INTERNAL_ERROR, e.str());
+    return false;
+    }
+
   bool needCxx98 = false;
   bool needCxx11 = false;
   bool needCxx14 = false;
@@ -5339,7 +5381,7 @@ bool cmMakefile::HaveCxxStandardAvailable(cmTarget const* target,
   const char *existingCxxStandard = target->GetProperty("CXX_STANDARD");
   if (!existingCxxStandard)
     {
-    existingCxxStandard = this->GetDefinition("CMAKE_CXX_STANDARD_DEFAULT");
+    existingCxxStandard = defaultCxxStandard;
     }
 
   if (std::find_if(cmArrayBegin(CXX_STANDARDS), cmArrayEnd(CXX_STANDARDS),