Becca Royal-Gordon e04ec5a7d9 Weaken some type checks for @preconcurrency decls ...

In an async context, trying to pass a non-`@Sendable` function to an `@Sendable` parameter or trying to assign a `@MainActor` method to a non-`@MainActor`-typed variable were hard errors. We now think that this a mistake for `@preconcurrency` APIs in Swift 5 mode, as it hinders retroactive adoption of `@Sendable` and `@MainActor` by libraries.

This PR weakens these errors to warnings *only* when the decl which contains the attribute in its type signature is `@preconcurrency` and *only* when in Swift 5 mode (with or without -warn-concurrency). For non-`@preconcurrency` decls, it is still an error.

Fixes <rdar://88703266>.

2022-02-15 13:35:55 -08:00

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