hpx::make_continuation requires input and output to be the same

[TheConstructor]

Today we encountered conversion-errors when trying to chain actions with different input and output types. Let me illustrate the issue with a short example:

boost::int32_t times2(boost::int32_t i) { return i * 2; }
HPX_PLAIN_ACTION(times2);    // defines times2_action

std::string my_to_string(boost::int32_t i) { return std::to_string(i); }
HPX_PLAIN_ACTION(my_to_string);    // defines to_string_action

boost::int32_t result = hpx::async_continue(
        times2_action(), 
        hpx::make_continuation(my_to_string_action()), 
        hpx::find_here(), 
        42).get();
hpx::cout << result << std::endl;   // will print: 3422212

Clearly the actions can be called in the given sequence, but the result should be std::string and not boost::int32_t and it should print 84.

Please provide a way to chain actions where input and output have different types.

[TheConstructor]

Mitigation requires an action that consumes hpx::future, as it seems that then requires the next action to consume futures while hpx::make_continuation does not.

boost::int32_t times2(boost::int32_t i) { return i * 2; }
HPX_PLAIN_ACTION(times2);    // defines times2_action

std::string my_to_string(hpx::future<boost::int32_t> i) { return std::to_string(i); }
HPX_PLAIN_ACTION(my_to_string);    // defines to_string_action

std::string result = hpx::async(times2_action(), hpx::find_here(), 42).then(
        hpx::util::bind(my_to_string_action(), 
                        hpx::find_here(), 
                        hpx::util::placeholders::_1)).get();
hpx::cout << result << std::endl;   // will print: 84

[hkaiser]

My first investigation shows that this is a bug in the return type calculation performed by make_continuation. This should be easily fixable.

[hkaiser]

@TheConstructor Please see branch 'fixing_1615' which should fix this issue. Please report back whether it solves your problem.

[TheConstructor]

I can confirm that this solves our problem with make_continuation.

[hkaiser]

This will be fixed by merging #1619