[Pitch] Custom Metadata Attributes

[bbrk24]

I don't think this is viable. At least, I don't like the arbitrary choice that has to be be made for types like

@Flag
struct MyType<T: AsyncSequence> { ... }

This is the same problem I brought up in the Reflection thread @filip-sakel linked above:

@bbrk24

I was thinking of [Void: Void], but I guess there's not always going to be a good equivalent -- what about AsyncLineSequence for example? Would I have to say something like AsyncLineSequence<TaskGroup<UInt8>>? There is no AnyAsyncSequence<T> type, and the AsyncSequence protocol doesn't currently support the generic syntax.

Further, it may not even always be possible to fulfill the archetype:

protocol NotImplemented {}

@Flag
struct CantUseMe<T: NotImplemented> {}