Add isPromiseType.

[heathkit]

Returns true if a type is a union type that includes Promise, or a type
that extends Promise.

[ajafff]

Thank you for the PR @heathkit.

I'm reluctant to add this function as is, because it's a very limited approach. This looks a lot like the code from TSLint's await-promise rule.
Let's ignore the licensing and copyright difficulties for now.
I'm wondering what your use case is and what you are trying to achieve with this function?

I'm asking because in the meantime I wrote two different implementations in another project. They are more spec compliant than this version. Both of them have a different purpose:

This one checks if a type that is already used with await could be a PromiseLike:
https://github.com/ajafff/wotan/blob/7adec1ac59fd9318f3e4f8cb43d373cb271af018/src/rules/await-promise.ts#L41-L58

This one checks if a type is really a PromiseLike by checking for a then method that takes a callback as first parameter:
https://github.com/ajafff/wotan/blob/7adec1ac59fd9318f3e4f8cb43d373cb271af018/src/rules/try-catch-return-await.ts#L92-L119

If I wanted to add one of these functions to tsutils, it would definitely be the second.

[heathkit]

It's kind of a long story, but I'm trying to deal with the Selnium WebDriver promise manager being removed (see SeleniumHQ/selenium#2969).

We have a lot of code that used to be synchronous, for which we'll now need to ask people to put await in all the right places. We're worried that knowing which parts of WebDriver's API are now async isn't intuitive, especially for people who are used to the synchronous API. Also, a missing await in a WebDriver test can be painful to debug. We thought we'd save people some heartache by making a check that will warn if there are promises in a given block that are unused - not passed to a function, awaited, or assigned to a variable.

Either of those functions seem fine. I'm just trying to catch promises that aren't awaited, but probably should be.

[ajafff]

promises in a given block that are unused - not passed to a function, awaited, or assigned to a variable.

That sounds like you want to use TSLint's no-floating-promises, which does exactly what you want, instead of writing your own rule.

Anyways, I'll add my implementation of isPromiseLike (from my second link above) to the next version of this library.