[llvm] r230010 - Make the static instance of None just const.

[Justin Bogner]

David Blaikie <dblaikie at gmail.com> writes:
> On Fri, Feb 20, 2015 at 5:16 AM, Benjamin Kramer <benny.kra at googlemail.com>
> wrote:
>
>     Author: d0k
>     Date: Fri Feb 20 07:16:05 2015
>     New Revision: 230010
>    
>     URL: http://llvm.org/viewvc/llvm-project?rev=230010&view=rev
>     Log:
>     Make the static instance of None just const.
>    
>     This way there shouldn't be any unused variable warnings.
>    
>     Modified:
>         llvm/trunk/include/llvm/ADT/None.h
>    
>     Modified: llvm/trunk/include/llvm/ADT/None.h
>     URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/
>     None.h?rev=230010&r1=230009&r2=230010&view=diff
>     ==========================================================================
>     ====
>     --- llvm/trunk/include/llvm/ADT/None.h (original)
>     +++ llvm/trunk/include/llvm/ADT/None.h Fri Feb 20 07:16:05 2015
>     @@ -20,7 +20,7 @@ namespace llvm {
>      /// \brief A simple null object to allow implicit construction of
>     Optional<T>
>      /// and similar types without having to spell out the specialization's
>     name.
>      enum class NoneType { None };
>
> At this point NoneType could just be an opaque/trivial struct instead of an
> enum class?
>  
>
>     -static NoneType None;
>     +const NoneType None = None;
>
> Does this result in ODR violations? (either because None is not defined in the
> program at all and it is ODR used, or because this is a definition (?) and
> it's in multiple translation units? - I always forget which)

This should be fine. NoneType "satisfies the requirements for appearing
in a constant expression", so based on [basic.def.odr].3 I don't think
None will ever be odr-used. This should also mean that your idea of
making this constexpr should be strictly clearer and equivalent.