Doug Gregor b5759c9fd9 [Concurrency] Allow overload 'async' with non-async and disambiguate uses. ...

Allow an 'async' function to overload a non-'async' one, e.g.,

    func performOperation(_: String) throws -> String { ... }
    func performOperation(_: String) async throws -> String { ... }

Extend the scoring system in the type checker to penalize cases where
code in an asynchronous context (e.g., an `async` function or closure)
references an asychronous declaration or vice-versa, so that
asynchronous code prefers the 'async' functions and synchronous code
prefers the non-'async' functions. This allows the above overloading
to be a legitimate approach to introducing asynchronous functionality
to existing (blocking) APIs and letting code migrate over.

2020-09-08 16:51:10 -07:00

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