[SYCL] Fix method definition of sub_group_mask::group_ballot

[haroon26]

Closes #8201
provided default value for predicate argument in sub_group_mask::group_ballot definition

[haroon26]

I have checked the issue and found that there will be error to provide default argument to group_ballot definition.
If I just provide default argument to declaration of the method, another error generates which force to define method right after declaration.
Here, I want the opinion if we should go with finding a way to provide default argument to group_ballot, or should change the documentation accordingly?

[sergey-semenov]

@haroon26 If I understand correctly, the reason you get an error is because you're adding a default argument to a friend declaration. Adding a non-friend declaration earlier, outside sub_group_mask, and specifying the default argument there should do the trick.

[haroon26]

Hello @sergey-semenov! Thanks for your suggestion. But again there is a problem if we define the method before class. The defintion says:

template <typename Group>
detail::enable_if_t<std::is_same<std::decay_t<Group>, sub_group>::value,
                    sub_group_mask>
group_ballot(Group g, bool predicate = true);

And you can see that in the return type, we are actually using sub_group, which will be not declared at that point. So this will also cause error.

[sergey-semenov]

That can be resolved by adding a forward declaration of sub_group_mask as well.

[AlexeySachkov]

@haroon26, could you please also add a test for the change? Just a simple -fsyntax-only test which checks compilation would be enough

[steffenlarsen]

LGTM! Thank you, @haroon26 !