Discriminated unions over enum members don't error as expected when used with number literals

TypeScript Version: nightly (2.2.0-dev.20161203)

Code:

const enum E { Hey, Ho }

type A = {
	x: E.Hey;
	y: string;
}

type B = {
	x: E.Ho;
	y: number;
}

var zzz: A | B = { x: 1, y: "hello" }; // Doesn't error
var zzz: A | B = { x: 54, y: "hello" }; // Doesn't error

Expected behavior:
Should error.

Actual behavior:
Doesn't error.

For comparison:

type A = {
	x: 0;
	y: string;
}

type B = {
	x: 1;
	y: number;
}

var zzz: A | B = { x: 1, y: "hello" }; // Errors as expected
/*
'Type '{ x: 1; y: string; }' is not assignable to type 'A | B'.
  Type '{ x: 1; y: string; }' is not assignable to type 'B'.
    Types of property 'y' are incompatible.
      Type 'string' is not assignable to type 'number'.'
*/

var zzz: A | B = { x: 54, y: "hello" }; // Errors as expected
/*
'Type '{ x: 54; y: string; }' is not assignable to type 'A | B'.
  Type '{ x: 54; y: string; }' is not assignable to type 'B'.
    Types of property 'x' are incompatible.
      Type '54' is not assignable to type '1'.'
*/

I think it's rather about "feature" of enums - #11559

const enum E { Hey, Ho }
const a: E.Hey = 54 // // Doesn't error

I am seeing a related error in similar situation -

const enum kinds {
    first,
    second,
}

interface IFData {
    kind: kinds.first;
    data: string;
}

interface ISData {
    kind: kinds.second;
    data: number;
}

type Data = IFData | ISData;

function checker(input: Data) {
    const { kind, data } = input;
    switch (kind) {
        /**
         * Property 'length' does not exist on type 'string | number'.
         * Property 'length' does not exist on type 'number'.
         * ^ Above error on following case line
         */
        case kinds.first: return data.length;

        /**
         * Property 'length' does not exist on type 'string | number'.
         * Property 'length' does not exist on type 'number'.
         * ^ Above error on following case line
         */
        case kinds.second: return Math.abs(data);
    }
}

Discriminated unions with enum kinds do not seem to work as I expected. Can anyone suggest possible correction other than going back to literal string kinds ?

Sushruth Sastry (@sushruth)

Sushruth Sastry (@sushruth) narrowing doesn't work once the discriminant is picked off into its own variable via destructuring

Yeah, I kind of figured that. It would have been awesome if it did work.

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Discriminated unions over enum members don't error as expected when used with number literals #12647

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Discriminated unions over enum members don't error as expected when used with number literals #12647

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normalser commented on Dec 6, 2016

sushruth commented on Mar 24, 2019

RyanCavanaugh commented on Mar 26, 2019

sushruth commented on Mar 28, 2019

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