EDIT 2: This still surprises me...

   let promisedString: Promise<string> = new Promise<string>(
      resolve => {
        // Perfectly valid in TypeScript!
        resolve(undefined)
      }
    )

EDIT: no, I was wrong — Bluebird typings won't allow this behaviour but they force me to annotate both the binding and the Promise constructor call:

    let promisedString: Promise<string> = new Promise<string>(
      resolve => {
        resolve(42) // type error here, ok
      }
    )

That's surprising TS ships with such unsafe typings for ES2015 Promise API...

Hm... no.

First, note that this is with noImplicitAny activated. You can check this in TS playground.

Then if we inspect what typings TS have for ES2015 Promises[1] and for Bluebird[2], you can clearly see that those are identical in this case..

[1]: https://github.com/Microsoft/TypeScript/blob/master/lib/lib....

[2]: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/mast...