is duplicated by:

• SR-8595 Protocol extension on protocol with subclass where restriction doesn't apply : class semantics
• SR-8596 Existential of protocol with subclass where restriction is not substitutable
• SR-8676 Swift compiler doesn't infer "class only protocol" from generic where clause
• SR-8730 Illegal instruction: 4; While emitting SIL for getter
• SR-8815 Adding protocol restriction causes crash
• SR-9275 Crash when calling init on a protocol's metatype where the protocol is constrained to a type
• SR-9477 Dynamic-stack-buffer-overflow caused by where clause on protocol
• SR-7331 Diagnose protocol 'where' clause with constraints on 'Self'

Issue Description:

The following code fails to compile in a swift playground:

protocol Toggling where Self: Equatable {
  static var all: [Self] { get }
  func toggled() -> Self
}
extension Toggling {
  func toggled() -> Self {
    let current = Self.all.index(of: self) ?? 0
    let next = (current + 1) % Self.all.count
    return Self.all[next]
  }
}

Changing to simple conformance makes it work:

protocol Toggling : Equatable { .... }

The two should be equivalent.

Also, putting the constraint of Self: Equatable on the protocol extension (and removing it from the protocol declaration) also makes it work.

Having the Self requirement on both the protocol definition and extension correctly identifies that the constraint is redundant but also fails to compile.