[5.5][Async Refactoring] Split ambiguous (error + success) completion handler calls into success and error case

[ahoppen]

• Explanation: Previously, when a completion handler call had arguments for both the success and error parameters, we were always interpreting it as an error call. In case the error argument was an Optional, this could cause us to generate code that didn't compile, because we throwed the Error? or passed it to continuation.resume(throwing), both of which didn't work.
  We now generate an if let statement that checks if the error is nil. If it is, the error is thrown. Otherwise, we interpret the call as a success call and return the result.
• Scope: Async refactoring where completion handler call has an argument for both the success and the error parameter (e.g. completion(result, error) instead of completion(result, nil))
• Risk: Low (the code we were generating previously was in most cases not what the user wanted)
• Testing: Added several regression tests
• Issue: rdar://80318664
• Reviewer: @hamishknight (Hamish Knight) and @bnbarham (Ben Barham) on original PR [Async Refactoring] Split ambiguous (error + success) completion handler calls into success and error case #38227

Example 1 (convert to continuation)

Base Code

func test(completionHandler: (Int?, Error?) -> Void) {
  withoutAsyncAlternativeThrowing { (theValue, theError) in
    completionHandler(theValue, theError)
  }
}

Old Refactoring Result

func test() async throws -> Int {
  return try await withCheckedThrowingContinuation { continuation in
    withoutAsyncAlternativeThrowing { (theValue, theError) in
      continuation.resume(throwing: theError) // error: Argument type 'Error?' does not conform to expected type 'Error'
    }
  }
}

New Refactoring Result

func testThrowingContinuationRelayingErrorAndResult() async throws -> Int {
  return try await withCheckedThrowingContinuation { continuation in 
    withoutAsyncAlternativeThrowing { (theValue, theError) in
      if let error = theError {
        continuation.resume(throwing: error)
      } else {
        guard let theValue = theValue else {
          fatalError("Expected non-nil success argument 'theValue' for nil error")
        }
        continuation.resume(returning: theValue)
      }
    }
  }
}

Example 2 (convert to async/await)

Base Code

func test(completion: (String?, Error?) -> Void) {
  simpleErr() { (res, err) in
    completion(res, err)
  }
}

Old Refactoring Result

func test() async throws -> String {
  let res = try await simpleErr()
  throw <#err#>
}

New Refactoring Result

func test() async throws -> String {
  let res = try await simpleErr()
  return res
}

[ahoppen]

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