cmTarget: Allow populating COMPILE_FEATURES using generator expressions.

Delay validation of the content as a feature if it contains a
generator expression. It will be checked again at generate-time
after evaluation.

Help/prop_tgt/COMPILE_FEATURES.rst

@@ -5,3 +5,7 @@ Compiler features enabled for this target.
 
 The list of features in this property are a subset of the features listed
 in the :variable:`CMAKE_CXX_COMPILE_FEATURES` variable.
+
+Contents of ``COMPILE_FEATURES`` may use "generator expressions" with the
+syntax ``$<...>``.  See the :manual:`cmake-generator-expressions(7)` manual for
+available expressions.

Source/cmLocalGenerator.cxx

@@ -1459,10 +1459,8 @@ void cmLocalGenerator::AddCompileOptions(
       this->AppendFlagEscape(flags, *i);
       }
     }
-  if (const char* featureProp = target->GetProperty("COMPILE_FEATURES"))
-    {
   std::vector<std::string> features;
-    cmSystemTools::ExpandListArgument(featureProp, features);
+  target->GetCompileFeatures(features);
   for(std::vector<std::string>::const_iterator it = features.begin();
       it != features.end(); ++it)
     {
@@ -1471,7 +1469,6 @@ void cmLocalGenerator::AddCompileOptions(
       return;
       }
     }
-    }
   this->AddCompilerRequirementFlag(flags, target, lang);
 }
 

Source/cmMakefile.cxx

@@ -4521,6 +4521,11 @@ bool cmMakefile::
 AddRequiredTargetFeature(cmTarget *target, const std::string& feature,
                          std::string *error) const
 {
+  if (cmGeneratorExpression::Find(feature) != std::string::npos)
+    {
+    target->AppendProperty("COMPILE_FEATURES", feature.c_str());
+    return true;
+    }
   bool isCxxFeature = std::find_if(cmArrayBegin(CXX_FEATURES) + 1,
               cmArrayEnd(CXX_FEATURES), cmStrCmp(feature))
               != cmArrayEnd(CXX_FEATURES);

Source/cmTarget.cxx

@@ -2616,6 +2616,22 @@ void cmTarget::GetCompileDefinitions(std::vector<std::string> &list,
     }
 }
 
+//----------------------------------------------------------------------------
+void cmTarget::GetCompileFeatures(std::vector<std::string> &features) const
+{
+  assert(this->GetType() != INTERFACE_LIBRARY);
+  for(std::vector<cmTargetInternals::TargetPropertyEntry*>::const_iterator
+      si = this->Internal->CompileFeaturesEntries.begin();
+      si != this->Internal->CompileFeaturesEntries.end(); ++si)
+    {
+    cmSystemTools::ExpandListArgument((*si)->ge->Evaluate(this->Makefile,
+                                        "",
+                                        false,
+                                        this),
+                                      features);
+    }
+}
+
 //----------------------------------------------------------------------------
 void cmTarget::MaybeInvalidatePropertyCache(const std::string& prop)
 {

Source/cmTarget.h

@@ -545,6 +545,7 @@ public:
                          const std::string& config) const;
   void GetAutoUicOptions(std::vector<std::string> &result,
                          const std::string& config) const;
+  void GetCompileFeatures(std::vector<std::string> &features) const;
 
   bool IsNullImpliedByLinkLibraries(const std::string &p) const;
   bool IsLinkInterfaceDependentBoolProperty(const std::string &p,

Tests/CompileFeatures/CMakeLists.txt

@@ -22,3 +22,8 @@ add_executable(CompileFeatures main.cpp)
 set_property(TARGET CompileFeatures
   PROPERTY COMPILE_FEATURES "cxx_auto_type"
 )
+
+add_executable(GenexCompileFeatures main.cpp)
+set_property(TARGET GenexCompileFeatures
+  PROPERTY COMPILE_FEATURES "$<1:cxx_auto_type>;$<0:not_a_feature>"
+)

Tests/RunCMake/CompileFeatures/NotAFeatureGenex-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CompileFeatures/NotAFeatureGenex-stderr.txt

@@ -0,0 +1,2 @@
+CMake Error in CMakeLists.txt:
+  Specified unknown feature "not_a_feature" for target "somelib".

Tests/RunCMake/CompileFeatures/NotAFeatureGenex.cmake

@@ -0,0 +1,3 @@
+
+add_library(somelib STATIC empty.cpp)
+set_property(TARGET somelib PROPERTY COMPILE_FEATURES "$<1:not_a_feature>")

Tests/RunCMake/CompileFeatures/RunCMakeTest.cmake

@@ -1,3 +1,4 @@
 include(RunCMake)
 
 run_cmake(NotAFeature)
+run_cmake(NotAFeatureGenex)