Assignability between number and enum type

[rkirov]

TypeScript Version: nightly (2.0.0-dev.201xxxxx)

Code

enum E {
  A,
  B
}

let n: number = 0;

let y: E = n;
let z: E|boolean = n;  // ERROR(9,5): : Type 'number' is not assignable to type 'boolean | E'.

Expected behavior:

Either disallow assigning number to E, reasonable if one considers E equivalent to 0 | 1. Or allow the assignment of number to E | boolean.

Actual behavior:

let z: E|boolean = n; // ERROR(9,5): : Type 'number' is not assignable to type 'boolean | E'.

[RyanCavanaugh]

Regression from 1.8 behavior

[rkirov]

Also for added mystery, changing the enum to single element enum E {A} removes the error. Admittedly, single member enums are not very common, but it might help you guys when debugging.

[sandersn]

I think this is caused by the switch to enums-as-unions, which would explain why the single-member enum doesn't have the bug.

[sandersn]

Also let z: E | boolean = 0 is fine too, so I think the 0 | 1 elements of the enum are incorrectly not assignable to number.

[sandersn]

isSimpleTypeRelatedTo has a special case that lets numbers be assignable to enums. Looks like it (or related code) should also have a special case for union types that contain enums.

[sandersn]

Fix is up at #10784