Generics do not preserve scoped union type of enums

[trshafer]

TypeScript Version: 3.0.0-dev.201xxxxx
I was unable to try this. This is attempted on current playground version (2.8 or 2.9, the playground does not specify)

Search Terms: NonNullable, union

Code

declare function test<T>(condition: T): NonNullable<T>;

enum A {
  ONE,
  TWO,
}

function thing(x: A.ONE) { // or a union type of some of the enum values.
  if (x === A.TWO) {
    // x cannot be A.TWO
  }

  const y = test(x);
  
  if (y === A.TWO) {
    // y can be A.TWO....
  }
}

Expected behavior:
y should not be assignable to A.TWO. y should remain A.ONE type.

Actual behavior:
y becomes type A instead of incoming A.ONE type.

Playground Link:
https://www.typescriptlang.org/play/#src=declare%20function%20test%3CT%3E(condition%3A%20T)%3A%20NonNullable%3CT%3E%3B%0D%0A%0D%0Aenum%20A%20%7B%0D%0A%20%20ONE%2C%0D%0A%20%20TWO%2C%0D%0A%7D%0D%0A%0D%0Afunction%20thing(x%3A%20A.ONE)%20%7B%0D%0A%20%20if%20(x%20%3D%3D%3D%20A.TWO)%20%7B%0D%0A%20%20%20%20%2F%2F%20x%20cannot%20be%20A.TWO%0D%0A%20%20%7D%0D%0A%20%20const%20y%20%3D%20test(x)%3B%0D%0A%20%20%0D%0A%20%20if%20(y%20%3D%3D%3D%20A.TWO)%20%7B%0D%0A%20%20%20%20%2F%2F%20y%20can%20be%20A.TWO....%0D%0A%20%20%7D%0D%0A%7D%0D%0A

Related Issues: #23884 #23046

Thanks for taking a look!

[mhegazy]

Literal types are not inferred unless there is a "marker" for the compiler e.g. constraint that suggests literal types are allowed.. so the type of y is A and not A.One.

you can get the desired behavior by defining test as:

declare function test<T extends number>(condition: T): NonNullable<T>;

[trshafer]

I realized this is unreleated to NonNullable and only relates to generics:

enum B {
  ONE,
  TWO
}

class A1<T> {
  constructor(readonly x: T) {}
}

class A2<T extends number> {
  constructor(readonly x: T) {}
}

console.log(new A1(B.ONE).x === B.TWO); // OK
console.log(new A2(B.ONE).x === B.TWO); // Error

Play link

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.