boolean is not assignable to false in type union discriminator field

[TomasHubelbauer]

TypeScript Version:

3.3.4000

Search Terms:

I used the search results provided when typing this issue's title and collected the results in the Related Issues section.

Code

type T1 = { flag: true; } | { flag: false; };
type T2 = true | false;
const flag = confirm('true | false');
const demo1: T1 = { flag };
const demo2: T2 = flag;

type T3 = { flag: true; value: 'a' | 'b' | 'c'; } | { flag: false; value: 'a' | 'b'; };
const value = 'a'; // Matches `'a' | 'b'`
const demo3: T3 = { flag, value: 'a' };

Expected behavior:

The code compiles with no errors.

Actual behavior:

The compiler displays an error on demo1 and demo3 lines, saying that boolean is not assignable to false. For this error to go away, one has to wrap the line in an if-else statement, passing a literal true in the if branch and false in the else branch.

The TypeScript compiler inference is probably not realizing that the rest of the union type members are compatible whichever way flag goes.

Playground Link:

TypeScript Playground

My repro repo

Related Issues:

• Type 'boolean' is not assignable to type 'true' when constructing discriminated union variant #10657 which says see Bug: Boolean type literals misbehaving #10432 which says fixed in Fix boolean literal types #10577 which says released in release-2.0
• Use of implicit union return type causes "'boolean' is not assignable to type 'false'" #19212 which seems to be about inheritance issues which I do not demonstrate
• true| false can not be used to create discriminated unions #17882 where the union members are not compatible between true and false which mine are

[TomasHubelbauer]

Nevermind, I have realized (of course only after posting this) that even though the union members are compatible between the true and false values of the union discriminator field, TypeScript still has no way of inferring the actual union member - it could be either, but it has no way of determining which one to take it for, so the code is rightfully rejected it seems like.