Proposal: Enable use Span in generic delegates

[AlexRadch]

Enable use Span in generic delegates.

For example as input parameter Func<Span<T>, T, int> and as output parameter Func<T, T, Span<T>>.

[HaloFour]

Why is it not sufficient to be able to define your own delegate types that accept/return Span<T>? It's not like you can take advantage of generic variance in these situations since Span<T> is a value type.

[AlexRadch]

@HaloFour Any struct is a value type. Span is ref struct.

[HaloFour]

@AlexRadch

Yes, ref structs are just structs with imposed limitations.

The CLR forbids struct generic type arguments to participate in variance, so you can't assign a Func<int> to a Func<object>, whereas you can assign a Func<string> to a Func<object>. So, with variance not possible, I don't see what the benefit would be to lift the imposed restriction that ref structs can't be generic type arguments where the generic type parameter is defined on a delegate. You're always free to define your own delegate, which is the appropriate route when the generic delegate families of Func<...> or Action<...> don't meet your need.

[AlexRadch]

Span can not be assigned to or from object, so Func<Span> can not be assigned to or from Func<object>. But it is not means that Func<Span> should be disabled as now.

[BladeWise]

@AlexRadch No, you can't.

            Func<int> getInt = () => 1;
            Func<object> getObj = getInt;

does not compile. Covariance is available either through interface or type inheritance, if I am not mistaken.

[svick]

@AlexRadch So what is the argument for enabling it? As far as I can tell, you haven't given any reason as for why this work should be done.

[AlexRadch]

@svick What argument for disabling it?

[AlexRadch]

For example why I should define separated delegate, where I can use Func?

    public delegate int IndexOfFunc<T>(Span<T> span, T value);

    public class MemoryExtensionsTests_IndexOfEqual<T>
    {
        public MemoryExtensionsTests_IndexOfEqual(IndexOfFunc<T> indexOf)
    }

Just use Func is smart

    public class MemoryExtensionsTests_IndexOfEqual<T>
    {
        public MemoryExtensionsTests_IndexOfEqual(Func<Span<T>, T, int> indexOf)
    }

[svick]

@AlexRadch That's not how this works. The current state is that it's disabled. You want to enable it, in other words, you're proposing a new language feature. And since features are a lot of work, they require justification. If you can'y provide that, I think the chance that will be accepted to the language is incredibly low.