relates to:

• SR-5148 Indirect enum creates wrong case if associated type is an enum with a single case

Issue Description:

I have simple struct which works as configuration for network endpoint.
One problem is that without indirect keyword program crashes at runtime with cyclic metadata message. It's already reported as a bug in Swift JIRA.
Because Void is not decodable I've created this empty struct Nothing and conformed it to decodable.

struct Nothing: Decodable { }

struct EndpointConfiguration<Resource: Decodable> {
    indirect enum ResourceOrigin {
        case none(Resource)
        case json
    }
    let resourceType: ResourceOrigin
}
let config = EndpointConfiguration(
    resourceType: .none(Nothing())
)
print(config)

Prints:

EndpointConfiguration<Nothing>(resourceType: EndpointConfiguration<Nothing>.ResourceOrigin.json)

Which is very weird - it's not the case I specified I was thinking that it has to be something with memory layout and changed `Nothing` to this form:

struct Nothing: Decodable {
    private let x = 0
    init() { }
}

Now same usage prints:

EndpointConfiguration<Nothing>(resourceType: EndpointConfiguration<Nothing>.ResourceOrigin.none(Nothing(x: 0)))

Which is totally fine.

Minimal reproduction example:

struct S {}

enum E {
    indirect case a(S)
    case b
}

print(E.a(S()))

It's probably connected to S or Nothing having zero storage. With class it works as intended.