Deprecate object::borrowed/object::stolen

[jagerman]

Provides (via a dummy template parameter hack) inline definitions of object::borrowed/object::stolen.

The constexpr static instances can cause linking failures if the compiler doesn't optimize away the reference, as reported in #770.

There's no particularly nice way of fixing this in C++11/14: we can't inline definitions to match the declaration aren't permitted for non-templated static variables (C++17 does allows "inline" on variables, but that obviously doesn't help us.)

One solution that could work around it is to add an extra inherited subclass to object's hierarchy, but that's a bit of a messy solution and was decided against in #771 in favour of just deprecating (and eventually dropping) the constexpr statics.

Fixes #770.

[jagerman]

(Awaiting confirmation from #770 that this actually fixes it).

[jagerman]

One comment: In C++17 this could be fixed by adding just:

inline constexpr object::borrowed_t object::borrowed;
inline constexpr object::stolen_t object::stolen;

which is, of course, much nicer than the template hack, but seeing as we need the hack for 14/11 already, I don't see much advantage in using #ifdefs to provide the alternative in C++17 mode.

[dean0x7d]

This looks too heavyweight. I would just replace all usages of borrowed with borrowed_t{}. The borrowed definition itself might need to stay for backward compatibility (to be removed in v3.0 with other breaking changes).

[jagerman]

I'm fine with that. Do you mean heavyweight in terms of added compiler complexity, or something else? (I did check that this doesn't change sizeof(object)).

[dean0x7d]

I mean complex both for the compiler and for humans. The only advantage is avoiding _t{} in the syntax, but the cost is multiple + private inheritance, template tricks, and a separate C++17 version. Definitely not worth it when the goal is simple-as-can-be tag dispatch.

[jagerman]

Okay; updated the PR to deprecate borrowed/stolen and replace all internal use with borrowed_t{}/stolen_t{}.

[dean0x7d]

Looks good to me.

[wjakob]

Looks good -- merged!