right now, it is not marked Sendable in the standard library, and the compiler cannot infer Sendable because it is generic. is it safe to mark them Sendable, if their base Sequences are Sendable?
I’d expect FlattenSequence to be Sendable if Base is. Like most lazy sequences, it is really just a wrapper around the base. I wouldn’t make that assumption official, since some lazy sequences might use a singleton cache or something, but it’s usually how they work.
I’d definitely avoid retroactively conforming types to Sendable if you can help it. If it should be Sendable, someone should make that change in the Standard Library itself.
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