Subtype relations of existential metatypes are not consistent in the language.

[CrystDragon]

Description

For classes , if we can write code like let a: TypeA = xxxx, where the static type of xxxx is TypeB, it reflects the fact that TypeB is a subtype of TypeA (at least at compile time).

However, when dealing with existentials, the language seems to have inconsistent behaviors.

Reproduction

protocol Base {}
protocol Derived: Base {}
class C: Derived {}

// case 1
let derived: Derived.Type = C.self
print(C.self is Derived.Type)  // true, as expected
print(C.Type.self is Derived.Type.Type)  // compiler warning: 'is' test is always true  (1)
                                         // runtime: prints false                       (2)
let derividMeta: Derived.Type.Type = C.Type.self  // compiles fine                      (3)

The behaviors of (1), (2), (3) are not consistent.

// case 2:
let base: Base.Type = derived
print(Derived.self is Base.Type) // prints false, not as expected   (4)

Expected behavior

// case 1
Behaviors of (1), (2), (3) should be consistent, either they all fail or they or success.

// case 2
I am not sure about this, but if (4) could prints true, the language surface will be more consistent in my opinion.

Environment

• 1.87.3 Apple Swift version 5.9.2 (swiftlang-5.9.2.2.56 clang-1500.1.0.2.5)
• Target: arm64-apple-macosx14.0
• Xcode: Version 15.1 (15C65)

Additional information

No response

[AnthonyLatsis]

(2) looks like an issue with runtime casting.
(4) could only be true as Derived.self is Base.Protocol (or (any Derived).self is (any Base).Type in modern spelling) because Derived.self is a singleton metatype of type Derived.Protocol or (any Derived).Type, not an existential metatype as the one your is is checking against. I’m not sure if this kind of subtyping relation is supposed to be defined though.

cc @tbkka

[CrystDragon]

(4) could only be true as Derived.self is Base.Protocol (or (any Derived).self is (any Base).Type as we would write it today) because Derived.self is a singleton metatype of type Derived.Protocol ((any Derived).Type), not an existential metatype as the one your is is checking against.

cc @tbkka

Yes, thanks, I got it wrong. The line let base: Base.Type = derived can only imply that instances of any Derived.Type can be cast to any Base.Type.