is duplicated by:

• SR-9582 Change "can derive conformances" condition for CaseIterable

relates to:

• SR-8777 CaseIterable synthesis should work in the presence of a manual AllCases typealias
• SR-9582 Change "can derive conformances" condition for CaseIterable

Issue Description:

If I try and declare an enum that wants to synthesize CaseIterable conformance but I break that by declaring the AllCases typealias myself, I get a confusing error when I try and access the allCases property.

enum Foo: CaseIterable {
    typealias AllCases = Set<Foo>
    case one, two, three
}
dump(Foo.allCases)

produces

<unknown>:0: error: 'allCases' is inaccessible due to 'internal' protection level
<unknown>:0: note: 'allCases' declared here
unnamed.swift:1:6: error: type 'Foo' does not conform to protocol 'CaseIterable'
enum Foo: CaseIterable {
     ^
unnamed.swift:1:6: error: protocol 'CaseIterable' is broken; cannot derive conformance for type 'Foo'
enum Foo: CaseIterable {
     ^

If I mark the enum as public instead, then the first error becomes the even more broken

<unknown>:0: error: 'allCases' is inaccessible due to '<<ERROR>>' protection level