Demonstrates a protocol which can help with overriding protocol-witness style dependencies, adding expectations when called.

Overrideable.swift

protocol Overrideable {}

extension Overrideable {
  mutating func `override`<Input: Equatable, Output>(
    method: WritableKeyPath<Self, @Sendable (Input) async throws -> Output>,
    expectedInput: Input,
    returning value: Output,
    name: String
  ) {
    let expectation = expectation(description: name)
    let capturedMethod = self[keyPath: method]
    self[keyPath: method] = { input in
      if input == expectedInput {
        expectation.fulfill()
        return value
      } else {
        return try await capturedMethod(input)
      }
    }
  }
}

extension Overrideable {
  mutating func `override`<each Input: Equatable, Output>(
    method: WritableKeyPath<Self, @Sendable (repeat each Input) async throws -> Output>,
    expectedInput: (repeat each Input),
    returning value: Output,
    name: String
  ) {
    let expectation = expectation(description: name)
    let capturedMethod = self[keyPath: method]
    
    // Cannot assign through subscript: 'self' is immutable
    self[keyPath: method] = { (inputs: (repeat each Input)) in
      let equal = isEqual(lhs: expectedInput, rhs: inputs)
      if equal {
        expectation.fulfill()
        return value
      } else {
        // Cannot pass value pack expansion to non-pack parameter of type 'repeat each Input'
        return try await capturedMethod(inputs)
      }
    }
  }
}

private func isEqual<each Element: Equatable>(lhs: (repeat each Element), rhs: (repeat each Element)) -> Bool {
  for (left, right) in repeat (each lhs, each rhs) {
    guard left == right else { return false }
  }
  return true
}

// Substitute for XCTest expectation
struct Expectation {
  var name: String

  func fulfill() {
    print("Fulfilled expectation \(name)")
  }
}

func expectation(description: String) -> Expectation {
  let expectation = Expectation(name: description)
  print("Created expectation")
  return expectation
}