A function that asserts that an expression throws a given error

AssertThrowsError.swift

/**
 *  Copyright (c) John Sundell 2017
 *  Licensed under the MIT license
 */

import Foundation
import XCTest

/**
 *  Assert that an expression throws a given error
 *
 *  - parameter expression: The expression that should throw an error
 *  - parameter errorExpression: An expression resulting in an error that should be thrown
 *  - parameter file: The file in which the assert should take place (automatically inferred)
 *  - parameter line: The line number at which the assert should take place (automatically inferred)
 *
 *  Usage: `assert(try myFunction(), throwsError: MyError.anError)`
 */
func assert<E: Error>(_ expression: @autoclosure () throws -> Void,
                      throwsError errorExpression: @autoclosure () -> E,
                      file: StaticString = #file,
                      line: UInt = #line) where E: Equatable {
    do {
        try expression()
        XCTFail("Expected expression to throw", file: file, line: line)
    } catch let thrownError as E {
        let expectedError = errorExpression()

        XCTAssert(thrownError == expectedError,
                  "Incorrect error thrown. \(thrownError) is not equal to \(expectedError)",
                  file: file,
                  line: line)
    } catch {
        XCTFail("Invalid error thrown: \(error)", file: file, line: line)
    }
}