The original proposal has lots of caveats, justifications, and pre-empts many comments and questions that people seem to be asking here, so I'd recommend people to take a look at that instead: https://github.com/giltayar/proposal-types-as-comments/

Also the announcement: https://devblogs.microsoft.com/typescript/a-proposal-for-typ...

And HN discussion: https://news.ycombinator.com/item?id=30618681