Doug Gregor 37d71f362e Add StrictSendableMetatypes to require Sendable requirements on metatypes ...

Introduce a new experimental feature StrictSendableMetatypes that stops
treating all metatypes as `Sendable`. Instead, metatypes of generic
parameters and existentials are only considered Sendable if their
corresponding instance types are guaranteed to be Sendable.

Start with enforcing this property within region isolation. Track
metatype creation instructions and put them in the task's isolation
domain, so that transferring them into another isolation domain
produces a diagnostic. As an example:

    func f<T: P>(_: T.Type) {
      let x: P.Type = T.self
      Task.detached {
        x.someStaticMethod() // oops, T.Type is not Sendable
      }
    }

2025-02-12 20:21:57 -08:00

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