Eric Niebler wrote:
Here's the surprising part:

Jeffrey Lee Hellrung, Jr. wrote:
> - If F::result_type exists, return F::result_type;
> - Else:
> - If BOOST_NO_CXX11_DECLTYPE, return F::result< F ( Args... ) >
> - Else, if F::result<> exists, return F::result< F ( Args... ) >
> - Else return decltype( declval<F>() ( declval< Args >()... ) )

The way I read this is that F::result_type (and F::result<>) is taking
precedence over decltype. AFAICT, you're suggesting that result_of *not*
compute the actual return type of a function object, but rather return
the declared result type, even if it's wrong.

In moving to a decltype-based result_of, we discovered just how many
function objects in boost were lying about their return types via the
TR1 result_of protocol. Now that we have decltype, we can get the right
answer always. I would be *immensely* surprised if result_of ever got
the wrong answer on a compiler that had decltype.

I understood the suggestion to apply when compilers had pre-3276 decltype -- so the above wouldn't apply to latest clang, nor to other compilers once they support incomplete types.

And btw, I don't think lambdas are required to have a nested result_type
typedef, are they? So what benefit does changing the protocol have?
Wouldn't you still need a wrapper anyway?

No, they're not required. That's the heart of the issue -- if result_of were modified (only for lacking compilers) to fall back to decltype, then the lack of result_type wouldn't require special handling by the user.

Thanks,
Nate