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.

5 files changed, 11 insertions, 5 deletions

diff --git a/Help/manual/cmake-developer.7.rst b/Help/manual/cmake-developer.7.rst
index 65b3a72..e18250c 100644
--- a/Help/manual/cmake-developer.7.rst
+++ b/Help/manual/cmake-developer.7.rst
@@ -80,7 +80,9 @@ When adding the first supported feature to a particular CompilerId, it is
 necessary to list support for all features known to cmake (See
 :variable:`CMAKE_C_COMPILE_FEATURES` and
 :variable:`CMAKE_CXX_COMPILE_FEATURES` as appropriate), where available for
-the compiler.
+the compiler.  Furthermore, set ``CMAKE_<LANG>_STANDARD_DEFAULT`` to the
+default language standard level the compiler uses, or to the empty string
+if the compiler has no notion of standard levels (such as ``MSVC``).
 
 It is sensible to record the features for the most recent version of a
 particular CompilerId first, and then work backwards.  It is sensible to
diff --git a/Help/prop_tgt/CXX_STANDARD.rst b/Help/prop_tgt/CXX_STANDARD.rst
index 6329e34..65b30ec 100644
--- a/Help/prop_tgt/CXX_STANDARD.rst
+++ b/Help/prop_tgt/CXX_STANDARD.rst
@@ -5,7 +5,8 @@ The C++ standard whose features are requested to build this target.
 
 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=gnu++11`` to the compile line.
+flag such as ``-std=gnu++11`` to the compile line.  For compilers that
+have no notion of a standard level, such as MSVC, this has no effect.
 
 Supported values are ``98``, ``11`` and ``14``.
 
diff --git a/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst b/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst
index ac103a4..4e24e5e 100644
--- a/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst
+++ b/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst
@@ -7,7 +7,8 @@ If this property is set to ``ON``, then the value of the
 :prop_tgt:`CXX_STANDARD` target property is treated as a requirement.  If this
 property is ``OFF`` or unset, the :prop_tgt:`CXX_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
-not available.
+not available.  For compilers that have no notion of a standard level, such as
+MSVC, this has no effect.
 
 See the :manual:`cmake-compile-features(7)` manual for information on
 compile features.
diff --git a/Help/prop_tgt/C_STANDARD.rst b/Help/prop_tgt/C_STANDARD.rst
index 88ca926..3aa74af 100644
--- a/Help/prop_tgt/C_STANDARD.rst
+++ b/Help/prop_tgt/C_STANDARD.rst
@@ -5,7 +5,8 @@ The C standard whose features are requested to build this target.
 
 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=gnu11`` to the compile line.
+flag such as ``-std=gnu11`` to the compile line.  For compilers that
+have no notion of a standard level, such as MSVC, this has no effect.
 
 Supported values are ``90``, ``99`` and ``11``.
 
diff --git a/Help/prop_tgt/C_STANDARD_REQUIRED.rst b/Help/prop_tgt/C_STANDARD_REQUIRED.rst
index a7304f4..743d568 100644
--- a/Help/prop_tgt/C_STANDARD_REQUIRED.rst
+++ b/Help/prop_tgt/C_STANDARD_REQUIRED.rst
@@ -7,7 +7,8 @@ If this property is set to ``ON``, then the value of the
 :prop_tgt:`C_STANDARD` target property is treated as a requirement.  If this
 property is ``OFF`` or unset, the :prop_tgt:`C_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
-not available.
+not available.  For compilers that have no notion of a standard level, such as
+MSVC, this has no effect.
 
 See the :manual:`cmake-compile-features(7)` manual for information on
 compile features.