Would be sibling to ServiceSubscriberInterface.
Could be added to ServiceLocator, to provide reflection on the inner of the locator.

ServiceProviderInterface::getProvidedService() would return an array, keyed by service names, and valued by service types. Special value "?" would mean "mixed" == "dunno" (or null? but this'd mean isset() wouldn't work on the array.)
The implementation on ServiceLocator would just use reflection on the closures and return their return-type hint.

The benefit of using ? for unknown types is that if one want to know if such a type can be null, then the logic is invariably '?' === $type[0]. Any other convention would require more code.