比较单元测试中的 swift Type 值 - XCTAssertEqual vs ==

Question

I'm trying to compare swift Type values in my unit tests and noticed that XCTAssertEqual fails to compile whereas comparing with == compiles fine.

XCTAssertEqual(MyStruct.self, MyStruct.self) --> Fails to compile with error "Global function 'XCTAssertEqual( : :_:file:line:)' requires that 'MyStruct.Type' conform to 'Equatable'"

XCTAssertTrue(MyStruct.self == MyStruct.self) --> Compiles fine

I would like to understand what is the difference between these two comparisons.

Answer 1

XCTAssertEqual requires that its arguments conform to Equatable . MyStruct.Type is a meta type, which, like all meta types, does not conform to Equatable , so MyStruct.self cannot be used as an argument to XCTAssertEqual .

However, the == operator is defined for all meta types. This is why you can use == on them. This is a "special case" implemented here .

"But isn't == one of the requirements of the requirements of Equatable ?" you might say. Yes, but that doesn't mean types who implements == automatically conform to Equatable . The converse is true though: every type that conforms to Equatable must implement == .