Regression: Operators in protocol extensions doesn't compile in Swift 6.1 and Swift 6.2 snapshots when the parameters are generalized to take any instance satisfying the protocol and not just Self

[TheHarcker]

Description

No response

Reproduction

public protocol EqualById {
    var id: String { get }
}

extension EqualById {
    public static func ==(lhs: EqualById, rhs: EqualById) -> Bool {
        return lhs.id == rhs.id
    }
}

Which can be reduced to:

public protocol EqualById {}
extension EqualById {
    public static func ==(lhs: EqualById, rhs: EqualById) -> Bool{
        true
    }
}

These examples compile successfully on Swift 6.0.3, but compilation fails on the swift-6.1-DEVELOPMENT-SNAPSHOT-2025-01-17-a.xctoolchain and swift-DEVELOPMENT-SNAPSHOT-2025-01-10-a.xctoolchain snapshots with the diagnostic:

 4 | 
 5 | extension EqualById {
 6 |     public static func ==(lhs: EqualById, rhs: EqualById) -> Bool{
   |                        `- error: member operator '==' of protocol 'EqualById' must have at least one argument of type 'Self'
 7 |         return lhs.id == rhs.id
 8 |     }

Expected behavior

I expected the code to compile on all toolchains as Self conforms to EqualById.

Environment

The code compiles with swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1) Target: arm64-apple-macosx15.0

The codes doesn't compile with Apple Swift version 6.1-dev (LLVM 38a3fc37c5f5e7a, Swift eedae18ae4df60a) Target: arm64-apple-macosx15.0 and Apple Swift version 6.2-dev (LLVM 81859ac55f8d09a, Swift 8ec8a1229a7ea14) Target: arm64-apple-macosx15.0

Additional information

The problem is specific to protocol extensions, as the following examples doesn't compile using either toolchain:

protocol SomeProtocol {}
struct SomeType: SomeProtocol {
    static func ==(lhs: SomeProtocol, rhs: SomeProtocol) -> Bool {
        true
    }
}

protocol SomeProtocol {}
struct SomeType: SomeProtocol {}
extension SomeType {
    static func ==(lhs: SomeProtocol, rhs: SomeProtocol) -> Bool {
        true
    }
}

The following examples, where the parameters are further generalized, doesn't compile on Swift 6.0.3 either, suggesting that the behavior in Swift 6.1 and 6.2 may be intentional:

public protocol EqualById {}
extension EqualById {
    public static func ==(lhs: Any, rhs: Any) -> Bool {
        true
    }
}

public protocol EqualById: BinaryInteger {}
extension EqualById {
    public static func ==(lhs: BinaryInteger, rhs: BinaryInteger) -> Bool{
        true
    }
}

public protocol EqualById {}
extension EqualById {
    public static func ==<T: EqualById>(lhs: T, rhs: T) -> Bool{
        true
    }
}

[stzn]

I traced the change history. It appears to be due to this change, which could be the correct fix.

[xedin]

cc @AnthonyLatsis