TypeErasingPropertyWrapper.swift

@propertyWrapper
struct ErasedToAnyPublisher<Upstream: Publisher> {
    var wrappedValue: AnyPublisher<Upstream.Output, Upstream.Failure> {
        upstream.eraseToAnyPublisher()
    }
    let upstream: Upstream
    init(upstream: Upstream) {
        self.upstream = upstream
    }
}

struct Foo {
    enum Error: Swift.Error {
        case bar
    }
    ...
    @ErasedToAnyPublisher<String, Error> var publisher: AnyPublisher<String, Error>
    init(a: A) {
        _publisher = ErasedToAnyPublisher(upstream: PassthrougSubject<String, Error>())
        ...
    }
    init(b: B) {
        _publisher = ErasedToAnyPublisher(upstream: PassthrougSubject<String, Error>())
        ...
    }
    func baz() {
        _publisher.upstream.send("")
    }
    ...
}

let foo = Foo(...)
foo.publisher.sink { print($0) } // publisher: AnyPublisher<String, Error>

// I'd be much happier if it was closer to

struct Foo {
    enum Error: Swift.Error {
        case bar
    }
    @ErasedToAnyPublisher var publisher = ErasedToAnyPublisher(upstream: PassthrougSubject<String, Error>())
    ...
    init(a: A) {
        ...
    }
    init(b: B) {
        ...
    }
    func baz() {
        _publisher.upstream.send("")
    }
    ...
}

let foo = Foo(...)
foo.publisher.sink { print($0) } // publisher: AnyPublisher<String, Error>