Futures should not require a default constructor

[eschnett]

future.hpp, line 248 says "return Result();". This requires a constructor that takes no arguments, which it shouldn't.

[K-ballo]

There are still pieces of code forcing the requirement of default constructibility for the result type.

[K-ballo]

Closing as duplicate of the more general #865

[krivenko]

What was the actual resolution of this issue?
Are return types of actions supposed to be default constructible?

Here is an extended version of tests/unit/actions/return_future.cpp

https://gist.github.com/krivenko/9e61c80e6adf4cd375af239f723dc752

test_plain_call_future_non_default_ctor() fails to compile with the following errors,

hpx.git/hpx/util/tuple.hpp:72:24: error: use of deleted function ‘non_default_ctor::non_default_ctor()’

hpx.git/hpx/lcos/base_lco_with_value.hpp:85:22: error: use of deleted function ‘non_default_ctor::non_default_ctor()’

[K-ballo]

@krivenko

What was the actual resolution of this issue?

This issue was resolved, Future<R> no longer requires R to be DefaultConstructible.

Are return types of actions supposed to be default constructible?

It's possible, action parameters and return types do have extra requirements, and implicit unwrapping means a remote invocation of an action returning Future<R> is treated as if it returned just R. I'd suggest creating a dedicated issue with your extended test case, for visibility.

[hkaiser]

@krivenko I don't have a quick fix for this - sorry, please give me some time to think it over.

[hkaiser]

@krivenko: please see #2847