throw-no-throw.swift

import Foundation
import XCTest

struct WompWomp: Error {

}

func myThrowingFunction() throws -> Int {
    throw WompWomp()
}

func myClosureFunction(_ closure: () -> Void) {
    closure()
}

class ThrowNoThrowTests: XCTestCase {

    func testExample() {
        myClosureFunction {
            XCTAssertEqual(myThrowingFunction(), 1) // Error: Call can throw but is not marked with 'try'
        }
    }
  
    func testExample2() {
        myClosureFunction { // Error: Invalid conversion from throwing function of type '() throws -> ()' to non-throwing function type '() -> Void'
            XCTAssertEqual(try myThrowingFunction(), 1)
        }
    }
  
    func testExample3() {
        myClosureFunction {
            do {
                try XCTAssertEqual(myThrowingFunction(), 1) // Test result: XCTAssertEqual failed: threw error "WompWomp()"
            } catch { // Warning: 'catch' block is unreachable because no errors are thrown in 'do' block
                print(error.localizedDescription)
            }
        }
    }
}

Maybe these examples also help:

extension ThrowNoThrowTests {
    func testExample4() {
        myClosureFunction {
            do {
                try XCTAssertEqualWithoutAutoclosure(myThrowingFunction(), 1)
            } catch {
                print(error.localizedDescription) // The operation couldn’t be completed. (__lldb_expr_14.WompWomp error 1.)
            }
        }
    }

    func testExample5() {
        myClosureFunction {
            XCTAssertEqualWithClosure({ try myThrowingFunction() }, { 1 }) // Test result: XCTAssertEqual failed: threw error "WompWomp()"
        }
    }
}

func XCTAssertEqualWithoutAutoclosure<T: Equatable>(_ lhs: T, _ rhs: T) {
    XCTAssertEqual(lhs, rhs)
}

func XCTAssertEqualWithClosure<T: Equatable>(_ lhs: () throws -> T, _ rhs: () throws -> T) {
    XCTAssertEqual(try lhs(), try rhs())
}