cmTarget: Add CXX_STANDARD_REQUIRED to control decay.

2 files changed, 16 insertions, 1 deletions

diff --git a/Help/prop_tgt/CXX_STANDARD.rst b/Help/prop_tgt/CXX_STANDARD.rst
index fb72f3e..fdd5aac 100644
--- a/Help/prop_tgt/CXX_STANDARD.rst
+++ b/Help/prop_tgt/CXX_STANDARD.rst
@@ -19,7 +19,8 @@ means that using:
 
 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.
+``-std=c++98`` flag if supported.  This "decay" behavior may be controlled
+with the :prop_tgt:`CXX_STANDARD_REQUIRED` target property.
 
 This property is initialized by the value of
 the :variable:`CMAKE_CXX_STANDARD` variable if it is set when a target
diff --git a/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst b/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst
new file mode 100644
index 0000000..f082805
--- /dev/null
+++ b/Help/prop_tgt/CXX_STANDARD_REQUIRED.rst
@@ -0,0 +1,14 @@
+CXX_STANDARD_REQUIRED
+---------------------
+
+Boolean describing whether the value of :prop_tgt:`CXX_STANDARD` is a requirement.
+
+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.
+
+This property is initialized by the value of
+the :variable:`CMAKE_CXX_STANDARD_REQUIRED` variable if it is set when a
+target is created.