あ、今気付いた。そうです。コードは↓。
import XCTest class SwiftCovariantCopyTests: XCTestCase { func testCopyWithoutAppend10M() { let cats: [Cat] = [Cat](repeating: Cat(), count: 10_000_000) measure { let animals: [Animal] = cats XCTAssertTrue(animals.last! is Cat) } } func testCopyWithAppend10M() { let cats: [Cat] = [Cat](repeating: Cat(), count: 10_000_000) measure { var animals: [Animal] = cats animals.append(Dog()) XCTAssertTrue(animals.last! is Dog) } } func testCopyWithoutAppend100M() { let cats: [Cat] = [Cat](repeating: Cat(), count: 100_000_000) measure { let animals: [Animal] = cats XCTAssertTrue(animals.last! is Cat) } } func testCopyWithAppend100M() { let cats: [Cat] = [Cat](repeating: Cat(), count: 100_000_000) measure { var animals: [Animal] = cats animals.append(Dog()) XCTAssertTrue(animals.last! is Dog) } } } class Animal {} class Cat: Animal {} class Dog: Animal {}