[edit, not quite what GP was referring to, but] At the minute, via [depending on context] a combination of generics/conditional types, unions/literals, type con

[edit, not quite what GP was referring to, but] At the minute, via [depending on context] a combination of generics/conditional types, unions/literals, type constructors, and [more recently] string templating you can get close in certain circumstances. It tends to be very fiddly though, and [IME] it's quite easy to cause TS to stop narrowing types properly. Someone else with more experience may be able to weight in -- despite using it for a few years I'd not really bothered to exercise some of the more expressive parts of the system until string templating experiments started popping up last year. This issue thread on the TS repo may be of interest: https://github.com/microsoft/TypeScript/issues/33014 https://github.com/microsoft/TypeScript/issues/33014