I can't reproduce this. Can you include a full playground, package, or project where this is happening, so that we can be sure we're building the same way as you?

Comment by Vitalii Korotkii (JIRA)

Sorry, here is the real code that reproduces my situation, did not notice that my getService method calls tryService.

struct MyError: Error { }
class OneService { }

func tryService<ServiceType>() throws -> ServiceType {
    print("\(ServiceType.self)")
    throw MyError()
}

func getService<ServiceType>() -> ServiceType? {
    return try? tryService()
}

let service: OneService? = getService()

In this example, given the change in behavior of try? Is the code working correctly and can the bug be closed?

Hm. That seems like an unintended consequence of SE-0230. @bjhomer, @xedin, any thoughts?

I agree that it's an unintended consequence, but I can guess why it's happening. The return type of tryService() is being inferred by its context, where the overall expression try? tryService() has to produce a ServiceType?. Prior to Swift 5, there was only one possible way for that to happen; tryService() had to produce a non-optional ServiceType so that the try? could add an extra layer of optional-ness.

Under Swift 5 with SE-0230, there are two options; tryService() could either produce a ServiceType or a ServiceType?. Either one satisfies the constraints. I suspect that the type checker is preferring the latter option because then try? doesn't have to add an implicit conversion. But it would make sense to prefer the less-optional alternative when possible.

At least, that's my guess.

Yes, it seems this way because `tryService` would like to return opened generic parameter as a result e.g. `$T0` which has to be converted to optional (e.g. $T1?) as a result of `try?` expression (that's why we add `$T0 conv $T1?` constraint), which means that only binding for `$T0` is `$T1?` which makes it - `ServiceType?` which doesn't involve any implicit conversions. It seems like we'd have to remodel how relationship of result type of try? and result of nested expression is represented.