Type guard does not work correctly with empty string type

[olderor]

Bug Report

🔎 Search Terms

type guard

🕗 Version & Regression Information

TypeScript 3.3.3+ (as per playground)

Please keep and fill in the line that best applies:

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Common "Bugs" That Aren't Bugs

⏯ Playground Link

Playground link with error

Playground link without error but with incorrect type

💻 Code

The compiled JavaScript version works correctly, but TypeScript gives errors:

type EmptyString = '' | null | undefined;

function isEmpty(value: string | EmptyString): value is EmptyString {
 return value === '' || value === null || value === undefined;
}

let test: string | null | undefined;

if (isEmpty(test)) {
 if (test === '') { // Compilation error since test: null | undefined and not test: '' | null | undefined
 console.log('Nope');
 }
}

Also, it's weird that when I remove null from the list of types, the error disappears, but the incorrect type is still shown:

type EmptyString = '' | undefined;

function isEmpty(value: string | EmptyString): value is EmptyString {
 return value === '' || value === undefined;
}

let test: string | undefined;

if (isEmpty(test)) {
 if (test === '') { // No errors, but the editor displays incorrect type.
 console.log('Nope');
 }
}

🙁 Actual behavior

TypeGuard ignores empty string and thinks the value can only be null or undefined.

🙂 Expected behavior

The type should include empty string. No errors expected.

[MartinJohns]

This looks very similar to #31156.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.