Allow type aliases to reference themselves in type argument positions

TypeScript Version: 3.8.0-dev.20191105

Search Terms: circular

Code

type A<T> = { x: T }
type R = A<R> // FAIL

type B = { x: B } // OK

Expected behavior: Both cases throw no error.

Actual behavior: Type R is rejected due to a circular reference.

Playground Link: Link

Related Issues: https://github.com/microsoft/TypeScript/pull/33050

Regression: https://github.com/microsoft/TypeScript/pull/33050#issuecomment-526637037

[AnyhowStep]

You know how averse someone is to OOP by their initial reaction to B = { x : B } =P

[AnyhowStep]

My initial reaction was that the recursion has to stop at some point. And that constructing an infinitely recursive type can’t really be performed without as casts at some point.

But I guess there’s also,

class SelfRecursive {
  self : SelfRecursive;
  constructor () {
    this.self = this;
  }
}

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So, I guess I just wasn’t open minded enough =P

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When I first saw the types in the original post, my head started screaming, “WHERE IS THE EXIT CONDITION!?”