Given an explicitly-nonisolated type such as
nonisolated struct S { }
all extensions of S were also being treated as nonisolated. This meant
that being implicitly nonisolated (i.e., when you're using nonisolated
default isolation) was different from explicitly-writing nonisolated,
which is unfortunate and confusing. Align the rules, such that an
extension of S will get default isolation:
extension S {
func f() { } // @MainActor if we're in main actor default isolation
}
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