duplicates:

• SR-263 Implement a general solution to prevent deadlocks when generic types rely on their own metadata recursively

is duplicated by:

• SR-5767 Unexpected cyclic metadata

relates to:

• SR-7604 cyclic metadata dependency detected, aborting
• SR-1686 Program with recursive class crashes because of “cyclic metadata dependency”
• SR-4611 Runtime crash with nested type used as superclass generic argument

Issue Description:

public struct Base<Element> {
  public struct Glue {
    enum Backing {
      case known(Element)
      case unknown(Int)
    }
    var backing: Backing
    
    static func unknown() -> Glue {
      return Glue(backing: .unknown(0))
    }
  }
}

let x: Base<Int>.Glue = .unknown() // CRASH: "GenericCache(0x1002df1c8): cyclic metadata dependency detected, aborting"