Test returns "Type of expression is ambiguous without more context" on func that throws

Question

I am writing a network layer, following a TDD approach.

I have a struct that I will be using to encode any parameters in my request.

I would like to assert that should a URL be missing / nil when invoking this encoder, the correct error is thrown.

struct URLParameterEncoder: ParameterEncoder {
    static func encode(urlRequest: inout URLRequest, with parameters: Parameters) throws {
        guard let url = urlRequest.url else { throw NetworkError.missingURL }
    }
}

I have written a test and passed in a URL which I believe should satisfy this test, however am getting an error when trying to run the test.

class URLParameterEncoderTests: XCTestCase {
    var sut: URLParameterEncoder!

    override func setUp() {
        sut = URLParameterEncoder()
    }

    func test_Encode_ThrowsMissingURLError_WhenURLIsNil() {
        let request = URLRequest(url: URL(string: "")!)
        let params: Parameters = [:]

        XCTAssertThrowsError(try sut.encode(urlRequest: &request, with: params)) { error in
            XCTAssertEqual(error as? NetworkError, NetworkError.missingURL)
        }
    }
}

In my XCTAssertThrowsError I am getting Type of expression is ambiguous without more context which I believe is being triggered around sut.encode

Additional code required to replicate

public typealias Parameters = [String: Any]

public protocol ParameterEncoder {
    static func encode(urlRequest: inout URLRequest, with parameters: Parameters) throws
}

public enum NetworkError: String, Error {
    case paramsAreNil = "Missing params"
    case encodingFailed = "Could not encode params"
    case missingURL = "Request URL is missing"
}

Answer 1

After adding more context – a question mark for optional chaining because sut is still an optional –

XCTAssertThrowsError(try sut?.encode(urlRequest: &request, with: params)) { error in ...

you get the real error

Static member 'encode' cannot be used on instance of type 'URLParameterEncoder'

It means that encode must be called on the type URLParameterEncoder.encode(...

---

However this raises a new error

Cannot pass immutable value as inout argument: 'request' is a 'let' constant

That means an inout parameter like request must be declared as variable: var request = ...