Document "do not use defaults in covered switch-over-enum" coding sta…

…ndard.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164389 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CodingStandards.rst

@@ -818,6 +818,24 @@ least one out-of-line virtual method in the class.  Without this, the compiler
 will copy the vtable and RTTI into every ``.o`` file that ``#include``\s the
 header, bloating ``.o`` file sizes and increasing link times.
 
+Don't use default labels in fully covered switches over enumerations
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``-Wswitch`` warns if a switch, without a default label, over an enumeration
+does not cover every enumeration value. If you write a default label on a fully
+covered switch over an enumeration then the ``-Wswitch`` warning won't fire
+when new elements are added to that enumeration. To help avoid adding these
+kinds of defaults, Clang has the warning ``-Wcovered-switch-default`` which is
+off by default but turned on when building LLVM with a version of Clang that
+supports the warning.
+
+A knock-on effect of this stylistic requirement is that when building LLVM with
+GCC you may get warnings related "control may reach end of non-void function"
+if you return from each case of a covered switch-over-enum because GCC assumes
+that the enum expression may take any representable value, not just those in
+the enumeration. To suppress this warning, use ``llvm_unreachable`` after the
+switch.
+
 Use ``LLVM_DELETED_FUNCTION`` to mark uncallable methods
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^