Features: Decay language flag if requested is not available.

Use the highest standard compile flags available if requested language
version is too new.

This supports use-cases like

 set(CMAKE_CXX_STANDARD 14)

 # Compiled with -std=c++11 with GNU 4.7, which has no -std=c++14
 # or equivalent flag
 add_executable(main main.cpp)

This can be used in combination with preprocessor defines which
communicate the availability of certain language features for
optional use.

Help/prop_tgt/CXX_STANDARD.rst

@@ -1,14 +1,26 @@
 CXX_STANDARD
 ------------
 
-The C++ standard whose features are required to build this target.
+The C++ standard whose features are requested to build this target.
 
-This property specifies the C++ standard whose features are required
+This property specifies the C++ standard whose features are requested
 to build this target.  For some compilers, this results in adding a
 flag such as ``-std=c++11`` to the compile line.
 
 Supported values are ``98`` and ``11``.
 
+If the value requested does not result in a compile flag being added for
+the compiler in use, a previous standard flag will be added instead.  This
+means that using:
+
+.. code-block:: cmake
+
+  set_property(TARGET tgt PROPERTY CXX_STANDARD 11)
+
+with a compiler which does not support ``-std=c++11`` or an equivalent
+flag will not result in an error or warning, but will instead add the
+``-std=c++98`` flag if supported.
+
 This property is initialized by the value of
 the :variable:`CMAKE_CXX_STANDARD` variable if it is set when a target
 is created.

Modules/Compiler/GNU-CXX.cmake

@@ -24,6 +24,8 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.3)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++0x")
 endif()
 
+set(CMAKE_CXX_STANDARD_DEFAULT 98)
+
 macro(cmake_record_cxx_compile_features)
   macro(_get_gcc_features std_version list)
     record_compiler_features(CXX "-std=${std_version}" ${list})

Source/cmLocalGenerator.cxx

@@ -2152,8 +2152,8 @@ AddCompilerRequirementFlag(std::string &flags, cmTarget* target,
     return;
     }
   std::string stdProp = lang + "_STANDARD";
-  const char *standard = target->GetProperty(stdProp);
-  if (!standard)
+  const char *standardProp = target->GetProperty(stdProp);
+  if (!standardProp)
     {
     return;
     }
@@ -2161,12 +2161,46 @@ AddCompilerRequirementFlag(std::string &flags, cmTarget* target,
   bool ext = target->GetPropertyAsBool(extProp);
   std::string type = ext ? "EXTENSION" : "STANDARD";
 
-  std::string compile_option =
-            "CMAKE_" + lang + std::string(standard)
-                     + "_" + type + "_COMPILE_OPTION";
-  if (const char *opt = target->GetMakefile()->GetDefinition(compile_option))
+  static std::map<std::string, std::vector<std::string> > langStdMap;
+  if (langStdMap.empty())
     {
-    this->AppendFlags(flags, opt);
+    // Maintain sorted order, most recent first.
+    langStdMap["CXX"].push_back("11");
+    langStdMap["CXX"].push_back("98");
+    }
+
+  std::string standard(standardProp);
+
+  std::vector<std::string>& stds = langStdMap[lang];
+
+  std::vector<std::string>::const_iterator stdIt =
+                                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)
+    {
+    defaultStdIt = std::find(stds.begin(), stds.end(), defaultStd);
+    assert(defaultStdIt != stds.end());
+    }
+  else
+    {
+    defaultStdIt = stds.end() - 1;
+    }
+
+  for ( ; stdIt <= defaultStdIt; ++stdIt)
+    {
+    std::string option_flag =
+              "CMAKE_" + lang + *stdIt
+                      + "_" + type + "_COMPILE_OPTION";
+
+    if (const char *opt = target->GetMakefile()->GetDefinition(option_flag))
+      {
+      this->AppendFlags(flags, opt);
+      return;
+      }
     }
 }