public extension PureYAML.Validation {
    /// A type-erased `Validation`, so checks specialised on different subject types live in one list, all
    /// sharing one `Document` (the validated tree's root type). It filters by runtime type: a
    /// `Validation<Subject, Document>` fires only on a `Subject` value, never on an optional of it (the
    /// `type(of:)` guard) or a value of any other type.
    struct AnyValidation<Document: Sendable>: Sendable {
        public let description: String
        private let applyErased: @Sendable (any Validatable, Path, Document) -> [ValidationError]

        public init<Subject: Validatable>(_ validation: Validation<Subject, Document>) {
            description = validation.description
            applyErased = { subject, path, document in
                guard let typed = subject as? Subject, type(of: subject) == Subject.self else {
                    return []
                }
                return validation.apply(ValidationContext(document: document, subject: typed, path: path))
            }
        }

        public func apply(
            to subject: any Validatable,
            at path: Path,
            in document: Document,
        ) -> [ValidationError] {
            applyErased(subject, path, document)
        }
    }
}