実際試してみたけど、やっぱ代入時にコピーだ・・・。

import XCTest class SwiftCovariantCopyTests: XCTestCase { func testCopyWithoutAppend1M() { measure { let cats: [Cat] = [Cat](repeating: Cat(), count: 1_000_000) let animals: [Animal] = cats XCTAssertTrue(animals.last! is Cat) } } func testCopyWithAppend1M() { measure { let cats: [Cat] = [Cat](repeating: Cat(), count: 1_000_000) var animals: [Animal] = cats animals.append(Dog()) XCTAssertTrue(animals.last! is Dog) } } func testCopyWithoutAppend10M() { measure { let cats: [Cat] = [Cat](repeating: Cat(), count: 10_000_000) let animals: [Animal] = cats XCTAssertTrue(animals.last! is Cat) } } func testCopyWithAppend10M() { measure { let cats: [Cat] = [Cat](repeating: Cat(), count: 10_000_000) var animals: [Animal] = cats animals.append(Dog()) XCTAssertTrue(animals.last! is Dog) } } } class Animal {} class Cat: Animal {} class Dog: Animal {}

Test Case '-[SwiftCovariantCopyTests.SwiftCovariantCopyTests testCopyWithoutAppend1M]' passed (0.592 seconds). Test Case '-[SwiftCovariantCopyTests.SwiftCovariantCopyTests testCopyWithAppend1M]' passed (0.790 seconds). Test Case '-[SwiftCovariantCopyTests.SwiftCovariantCopyTests testCopyWithoutAppend10M]' passed (4.162 seconds). Test Case '-[SwiftCovariantCopyTests.SwiftCovariantCopyTests testCopyWithAppend10M]' passed (6.556 seconds).

(edited)