I’ve cut my code down to the following simplest version to show the problem. I’m not sure why Swift is complaining.

Why does DatabaseGateway need to conform to protocol Something ???

struct Database {}

open class DatabaseGateway {
    
    let database: Database
    
    public init() {
        database = Database()
    }
}

struct FilterParameter<T> {}

protocol Something {
    associatedtype T: RawRepresentable where T.RawValue == String
    var filterParameter: FilterParameter<T> { get }
}

enum VoteTable {}

extension VoteTable {
    
    enum Field: String {
        case value = "value"
    }
}

final class User: DatabaseGateway, Something {
    let filterParameter = FilterParameter<VoteTable.Field>()
}

final class Poll: DatabaseGateway, Something {
    let filterParameter = FilterParameter<VoteTable.Field>()
}

struct VoteRecordFilter {
    
    let parameters: [Parameter]
    
    init(parameter: Parameter) {
        self.init(parameters: [parameter])
    }
    
    init(parameters: [Parameter]) {
        self.parameters = parameters
    }
    
    init<S: Something>(parameters: [S]) where S.T == VoteTable.Field {
        self.init(parameters: parameters.map({
            $0.filterParameter
        }))
    }
        
    typealias Parameter = FilterParameter<VoteTable.Field>
}

extension Poll {
    
    func voteSubmitted(user: User) async throws -> Bool {
        let filter = VoteRecordFilter(parameters: [self, user]) // Initializer 'init(parameters:)' requires that 'DatabaseGateway' conform to 'Something'
        return true
    }
}