Fix is_iterator for container algorithms

[dcbdan]

• Fix is_iterator so that SFINAE can work
• Call projection functions with invoke so that member functions
  can be passed as arguments.

[hkaiser]

That should probably be hpx::util::invoke

[hkaiser]

lgtm, thanks!

[AntonBikineev]

@hkaiser: just out of curiosity, what's benefit of using explicit quailification of hpx::util::invoke instead of using declarations in this case?

[hkaiser]

@hkaiser: just out of curiosity, what's benefit of using explicit quailification of hpx::util::invoke instead of using declarations in this case?

Using invoke allows for things like this:

struct A { 
    int foo() { return 42; }
};

std::vector<A> v(43);

parallel::for_each(
    seq, begin(v), end(v),
    [](int i) { cout << i; },
    &A::foo);

[K-ballo]

what's benefit of using explicit quailification of hpx::util::invoke instead of using declarations in this case?

@AntonBikineev It avoids ADL havoc, qualified vs unqualified is always about avoiding ADL. For instance, when std:: arguments are used both hpx::util::invoke and std::invoke are viable matches and the unqualified call is ambiguous. Generic libraries should always do qualified calls unless they explicitly want a call to reach a customization point.

[AntonBikineev]

@K-ballo: thanks for the point. What about this:
using hpx::util::invoke;
(invoke)(something);
/joke/