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?)

在链接的 TypeScript Playground 中重现该示例，并将 a && b 的推断类型与报告的运行时值进行比较。阅读相关 issue #32778，并跟踪编译器对 number 值进行的 truthiness narrowing；当 NaN 不再被缩小为 0，且预期类型 number | boolean 有回归测试覆盖时，即视为完成。