Number typeguards

[jods4]

TypeScript Version: 4.1.1-rc

Search Terms: Number typeguard

Code

let x = 42 as number | null;
Number.isFinite(x) && x.toString()

Expected behavior: No error nor warnings.
The following built-in Number functions should be defined as typeguards as they can only return true when their argument is a number:
isFinite, isInteger, isNaN, isSafeInteger.
They currently typed as simple boolean and hence do not modify TS flow analysis.

Actual behavior: TS2531, objet is possibly null

Playground Link: https://www.typescriptlang.org/play?ts=4.1.0-beta#code/DYUwLgBAHhC8EBYBMECGBnCA7ArgWwCMQAnCAH2x2GAG4AoAOXyOIDoBLdAMXa3bBAAKKAEoIAMnHRWYAPYBlMMV4BzQSJpA

Related Issues:

[MartinJohns]

Duplicate of #39090. Search terms used: isFinite

[jods4]

Indeed.