An AssertUnwrap method for use in Xcode 10, inspired by Xcode 11's `XCTUnwrap`

AssertUnwrap.swift

import XCTest

/// Attempts to unwrap an optional, failing the test if the value is nil. Your test method needs to be marked as `throws` to use this, and unwrap with:
/// `let value = try AssertUnwrap(optionalValue)`
///
/// Inspired by the upcoming `XCTUnwrap()` in Xcode 11.
///
/// - Parameters:
///   - value: Optional to unwrap
///   - message: If you want to override the default error message
/// - Returns: The non-optional value if present
/// - Throws: Ignore the error, throws just so we can stop execution of the assert method
internal func AssertUnwrap<Wrapped>(_ value: Wrapped?,
                                    message: String = "AssertUnwrap failed: Expected non-optional value was nil",
                                    file: StaticString = #file,
                                    line: UInt = #line) throws -> Wrapped {
    if let value = value {
        return value
    } else {
        XCTFail(message, file: file, line: line)
        throw AssertUnwrapError.nil
    }
}

/// Attempts to unwrap and cast an optional, failing the test if the value is nil or not of type `ExpectedType`. Your test method needs to be marked as `throws` to use this, and unwrap with:
/// `let value: MyType = try AssertUnwrap(optionalValue)`
///
/// Inspired by the upcoming `XCTUnwrap()` in Xcode 11.
///
/// - Parameters:
///   - value: Optional to unwrap
///   - message: If you want to override the default error message
/// - Returns: The non-optional value if present
/// - Throws: Ignore the error, throws just so we can stop execution of the assert method
internal func AssertUnwrap<Wrapped, ExpectedType>(_ value: Wrapped?,
                                                  message: String = "AssertUnwrap failed: Expected non-optional value was nil",
                                                  file: StaticString = #file,
                                                  line: UInt = #line) throws -> ExpectedType {
    if let value = value as? ExpectedType {
        return value
    } else {
        XCTFail(message, file: file, line: line)
        throw AssertUnwrapError.nil
    }
}


enum AssertUnwrapError: Error {
    case `nil`
}