调用 XCTAssertEqual() 时无法推断通用参数“T”

Question

The topic title is a compiler error related to the xCode Assert Equal test in the testPrimesUpTo100ShouldBe25() method.

One thing I noticed is that the call to the calculate() method in PrimeCalculator, when called from testPrimePerformance() and testPrimesUpTo100ShouldBe25(), only 'colours up' green in testPrimePerformance().

Full swift source file is below:

import XCTest

struct PrimeCalculator {
    static func calculate(upTo max: Int) -> [Int] {
        guard max > 1 else {
            return []
        }

        var sieve = [Bool](repeating: true, count: max)

        sieve[0] = false
        sieve[1] = false

        for number in 2 ..< max {
            if sieve[number] == true {
                for multiple in stride(from: number * number, to: sieve.count, by: number) {
                    sieve[multiple] = false
                }
            }
        }

        // collapse our results down to a single array of primes
        let primes = sieve.enumerated().compactMap { $1 == true ? $0 : nil }
        return primes
    }
}

class AsynchronousTests: XCTestCase {

    func testPrimePerformance() {
        measure {
            _ = PrimeCalculator.calculate(upTo: 1_000_000)
        }
    }

    func testPrimesUpTo100ShouldBe25() {
        // given
        let maximumCount = 100

        // when
        let progress = PrimeCalculator.calculate(upTo: maximumCount) {
            XCTAssertEqual($0.count, 25)
        }
        // then
        let predicate = NSPredicate(format: "@.completedUnitCount == %@", argumentArray: [progress, maximumCount])

        let expectation = XCTNSPredicateExpectation(predicate: predicate, object: progress)
        wait(for: [expectation], timeout: 10)
    }

    override func setUp() {
        // Put setup code here. This method is called before the invocation of each test method in the class.
    }

    override func tearDown() {
        // Put teardown code here. This method is called after the invocation of each test method in the class.
    }

    func testExample() {
        // This is an example of a functional test case.
        // Use XCTAssert and related functions to verify your tests produce the correct results.
    }

    func testPerformanceExample() {
        // This is an example of a performance test case.
        measure {
            // Put the code you want to measure the time of here.
        }
    }
}

Answer 1

Here

let progress = PrimeCalculator.calculate(upTo: maximumCount) {
    XCTAssertEqual($0.count, 25)
}

you pass a “trailing closure” to the calculate() method, which has two problems:

• First, that method takes only a integer argument, but not a closure argument. This is the actual problem.
• Second, the compiler can not determine the type of $0 from the context. This causes the compiler error message.

What you probably want is simply

let progress = PrimeCalculator.calculate(upTo: maximumCount)
XCTAssertEqual(progress.count, 25)