"This condition will always return 'false'" but returns true at runtime

TypeScript Version: 3.1.0-dev.20180818

Code

function sanityCheck(s: string): string {
	return s == 0 ? "I must be going insane" : "I'm fine";
}
console.log(sanityCheck(""));

Expected behavior:

Error message is 100% accurate.

Actual behavior:

src/a.ts:2:9 - error TS2367: This condition will always return 'false' since the types 'string' and 'number' have no overlap.

The == operator in JavaScript converts its arguments -- some developers apparently rely on this behavior (not naming names but I noticed this in real JavaScript code that I pasted into a TypeScript file).
The error message might recommend using an explicit conversion such as Number(s) to get this to compile in TypeScript.

CC @DanielRosenwasser

[DanielRosenwasser]

Yeah, we discussed == and != making the statement technically incorrect and decided to wait for the first bug report before addressing it.

[DanielRosenwasser]

I think just using the old error message is probably fine.

[Kingwl]

related #24991

[HZooly]

I got this error Operator '==' cannot be applied to types 'string' and 'number'. in Stackblitz.
https://stackblitz.com/edit/typescript-vwakj2

EDIT : Sorry, Typescript current version on Stackblitz shouldn't be 3.1.x

@HZooly That links to a document that doesn't use == anywhere?
Regardless, the solution is probably to be more explicit, and convert the string to Number(s) or the number to String(n), then use ===.

[HZooly]

@Andy-MS Sorry my snippet wasn't saved! Now it has been updated :)

[JohnPool]

I get the same error message for the following code: