Case: using type erasure

main.swift

protocol FooRepository {
    func foo() -> String
}

final class FooClient {
    static let shared = FooClient()
    
    private init() {}
}

extension FooClient: FooRepository {
    func foo() -> String {
        "Foo"
    }
}

final class FooInteractor {
    private let fooRepository: AnyFooRepository
    
    init(fooRepository: any FooRepository) {
        self.fooRepository = AnyFooRepository(fooRepository)
    }
    
    init(fooRepository: FooClient = .shared) {
        self.fooRepository = AnyFooRepository(fooRepository)
    }
}

class AnyFooRepository: FooRepository {
    
    let _foo: () -> String
    func foo() -> String {
        _foo()
    }
    
    init(_ base: FooRepository) {
        _foo = { base.foo() }
    }
}