[Phoenix V3] Add tr1_result_of specialization

[Michel Morin]

To make result_of and tr1_result_of equivalent, we have to add specialization of tr1_result_of. (Boost.Phoenix V3 already has specialization of result_of.)

Also, it would be nice to avoid specialization of result_of, when we use decltype-based result_of. (As for tr1_result_of, it should be specialized even when decltype-based result_of is used.)

So, instead of

template <...>
struct result_of<F()>
{
    typedef XXXX type;
};

we should write

#if !defined(BOOST_RESULT_OF_USE_DECLTYPE) || defined(BOOST_NO_DECLTYPE)
template <...>
struct result_of<F()>
{
    typedef XXXX type;
};
#endif

template <...>
struct tr1_result_of<F()>
{
    typedef XXXX type;
};

A quick grep said the following files specialize result_of.

• phoenix/core/actor.hpp
• phoenix/function/function.hpp

[Kohei Takahashi]

IMO, those specialization aim for returning non void type on TR1 result_of, so only tr1_result_of specialization is needed.

[Michel Morin]

Ah, result_of derives from tr1_result_of even for nullary calls. So the specialization of result_of is verbose.

Thanks for the correction!

[Kohei Takahashi]

Here is candidate patch for fixing this issue, but test is needed.

​https://github.com/boostorg/phoenix/commit/0d1af868ae86ff9ff1e791255af286a25ecf370e