Many times, TypeScript can infer the types of arguments you pass in a function call by looking at their definition. Consider the following example:

const promise = Promise.resolve(15);
f(promise);

In this case, TypeScript can infer the type of promise as Promise<number>. We can take advantage of this information to add type information to the first parameter of f. So given the following code as input:

function f(a) {
  return a;
}

const promise = Promise.resolve(15);
f(promise);

We should be able to discover the type of argument a correctly, and get the following result:

function f(a: Promise<number>) {
  return a;
}

const promise = Promise.resolve(15);
f(promise);

This PR adds experimental support for this feature. At this time, the code is still work-in-progress and is not yet ready to be merged.