NaN incorrectly narrows to 0

[osyrisrblx]

TypeScript Version: 3.9.0-dev.20200222

Search Terms: NaN Truthiness Falsy

Code

function bool(): boolean { return false; }
function number(): number { return NaN; }

let a = number();
let b = bool();

const x = a && b; // type = boolean | 0
console.log(x); // prints NaN (which is neither boolean, nor 0)

Expected behavior:
Type of x should be number | boolean.

Since NaN isn't a "unit type" like 0, we can't express NaN without using number.

Actual behavior:
Type of x is 0 | boolean.

Playground Link: playground link

Related Issues:
#32778 (Incorrectly marked as duplicate?)

[RyanCavanaugh]

See also #32277 - would need a NaN type to represent this

[jcalz]

Looks like people might be slightly talking past each other here. Soundness would require that a number which has been determined to be falsy should be narrowed to type 0 | typeof NaN.

So either:

• NaN should have its own numeric literal type, despite this comment in Number, enum, and boolean literal types #9407 saying we don't need it. This is a duplicate of NaN, Infinity and -Infinity not accepted in number literal types #15135, but maybe we should revisit it here?
• Falsy checks on number shouldn't narrow at all since 0 | typeof NaN is just number. That seems to be what this issue is specifically asking for.
• Everything stays as it is and we don't really care about soundness when it comes to NaN and truthiness.

[osyrisrblx]

• Falsy checks on number shouldn't narrow at all since 0 | typeof NaN is just number . That seems to be what this issue is specifically asking for.

Exactly this. If we can't have NaN as a type, then it shouldn't narrow for number.

I'd also say this isn't a "suggestion", but a soundness/type safety bug!

[zacnomore]

May I politely suggest changing the title to "NaN incorrectly narrows to 0" or something similar? I
think it would help ensure that this is correctly identified as a bug .