[SR-7495] Checking protocol conformance doesn't work

[swift-ci]

Previous ID	SR-7495
Radar	None
Original Reporter	domas (JIRA User)
Type	Bug
Status	Resolved
Resolution	Won't Do

Environment

Xcode 9.3 (9E145)
Swift version 4.1 (swiftlang-902.0.48 clang-902.0.37.1)
Target: x86_64-apple-darwin17.5.0
macOS 10.13.4 (17E199)

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: 723173a88503687fffbeb4aa3a8c991c

Issue Description:

Trying to check if type conforms to a protocol in an indirect way fails.

protocol P {}
struct T: P {}

// 0. Check with `is`
T.self is P.Type // true

// 1. Direct conformance check
func isOfTypeP(_ t: Any.Type) -> Bool {
    P.self // __lldb_expr_67.P.Protocol
    return t is P.Type
}
isOfTypeP(T.self) // true
 
// 2. Indirect conformance check
func isType<G>(_ t: Any.Type, subtypeOf _: G.Type) -> Bool {
    G.self // __lldb_expr_67.P.Protocol
    return t is G.Type
}
isType(T.self, subtypeOf: P.self) // false!!

Checking the superclass in the same way works as expected.

class S {}
class C: S {}

isType(C.self, subtypeOf: S.self) // true

[AnnaZaks]

cc @jckarter

[jckarter]

This is correct behavior. `P.self` is the concrete metatype for `P` itself, which has type `P.Protocol`, and not the type `P.Type`, which represents some type conforming to `P`. `T.self` is not exactly the type `P`, so the cast in `isType<G>` fails.

Something like this will work instead:

func isType<GType>(_ t: Any.Type, subtypeOf _: G) -> Bool {
    G.self // __lldb_expr_67.P.Protocol
    return t is G
}

isType(T.self, subtypeOf: P.Type.self)

[swift-ci]

Comment by Domas Nutautas (JIRA)

Thank you for a clarification! Your solution (with a small tweak) works perfectly 👍

func isType<G>(_ t: Any.Type, subtypeOf _: G.Type) -> Bool {
    G.self // __lldb_expr_67.P.Type.Protocol
    return t is G
}

isType(T.self, subtypeOf: P.Type.self) // true