NaN incorrectly narrows to 0

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:
https://github.com/microsoft/TypeScript/issues/32778 (Incorrectly marked as duplicate?)

リンク先の TypeScript Playground にある例を再現し、a && b の推論される型と報告された実行時の値を比較します。関連する issue #32778 を読み、number の値に対するコンパイラーの truthiness narrowing を追跡します。NaN が 0 に絞り込まれず、期待される型 number | boolean が回帰テストでカバーされれば完了です。