[swift-evolution] Fix "private extension" (was "Public Access Modifier Respected in Type Definition")

[Nevin Brackett-Rozinsky]

Two weeks ago I had a fairly strong opinion about “private extension”
behavior. After following this discussion, I now have no opinion on the
matter.

I would summarize the points on both sides as follows:

For the change:
• It is surprising to many people that members of a private extension are
implicitly fileprivate.
• There is currently no way to make an extension whose members default to
private.

Against the change:
• The proposal is source-breaking.
• The proposal makes “fileprivate” more common.
• A private extension and a (top-level) private type both currently have
implicitly fileprivate members. The proposal breaks that symmetry.

Notable questions:
• Currently “open” cannot be applied to an extension at all, should we
allow it?
• Might we ever want to allow nested (non-top level) extensions, and if so
how should access levels on them work?

Nevin
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