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Comparing swift Type values in unit tests - XCTAssertEqual vs ==

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.

Solution

  • 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 ==.