Swift Type Erase test playground.

TypeErase.swift

import Foundation

// MARK: - Entity

struct HogeEntity {
    let title: String
}

struct FugaEntity {
    let title: String
}

// MARK: - Protocol

protocol Repository {
    associatedtype Success
    
    func fetch(completion: ((Result<Success, Error>) -> Void)?)
}

// MARK: - Repositories

struct HogeRepository: Repository {
    typealias Success = HogeEntity
    
    func fetch(completion: ((Result<HogeEntity, Error>) -> Void)?) {
        guard let completion = completion else { return }
        completion(.success(HogeEntity(title: "Hoge success")))
    }
}

struct FugaRepository: Repository {
    typealias Success = FugaEntity
    
    func fetch(completion: ((Result<FugaEntity, Error>) -> Void)?) {
        guard let completion = completion else { return }
        completion(.success(FugaEntity(title: "Fuga success")))
    }
}

// MARK: - Type Erase

struct AnyRepository<SuccessType>: Repository {
    typealias Success = SuccessType
    
    private let _fetch: ((((Result<SuccessType, Error>) -> Void)?) -> Void)
    
    init<T: Repository>(_ base: T) where T.Success == SuccessType {
        _fetch = base.fetch
    }
    
    func fetch(completion: ((Result<SuccessType, Error>) -> Void)?) {
        _fetch(completion)
    }
}

// NOTE: これがやりたいけどできない
// `associatedType` で与えた型が解決していないためエラーが出る.
//let repository: Repository = HogeRepository()

let repository: AnyRepository<FugaEntity> = AnyRepository(FugaRepository())
repository.fetch { result in
    if case .success(let entity) = result {
        print(entity)
    }
}