[swift-evolution] Why doesn't Collection's SubSequence conform to Collection?

[Charles Srstka]

The following comment accompanies the declaration of the SubSequence associated type in the Collection protocol:

/// A sequence that represents a contiguous subrange of the collection's
/// elements.
///
/// This associated type appears as a requirement in the `Sequence`
/// protocol, but it is restated here with stricter constraints. In a
/// collection, the subsequence should also conform to `Collection`.
associatedtype SubSequence : IndexableBase, Sequence = Slice<Self>

The comments clearly state that the subsequence should conform to Collection, however, it is not declared as such. Why is this?

Charles

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