[Proposal] Generic version of nullable attributes aka NotNull<NotNull, MaybeNull>

[Orace]

With current existing attribute, one can write:

public interface Test
{
    // The returned value may be null.
    [return: MaybeNull]
    TOut GetFor<TIn, TOut>(TIn key);
}

But if the return value is a generic type, it's not possible do give nullability information for the type parameters.

public interface Test
{
    // useless
    // The returned value is a ValueTuple and can't be null anyway
    // How to mark value as MaybeNull ?
    [return: MaybeNull]
    (bool b, TOut value) TryGetFor<TIn, TOut>(TIn key);
}

This proposal addresses this.
We may add generic version of the nullable-attributes that accept 1 to 8 (16, 32 ?) generic parameter of a common nullable-attributes type.

This may allow to write:

public interface Test
{
    // The returned value is a ValueTuple and can't be null
    // but the TOut value can be.
    [return: NotNull<NotNull, MaybeNull>]
    (bool b, TOut value) TryGetFor<TIn, TOut>(TIn key);
}

Of course we first need generic attributes.