Bad reporting of unintentional comparison

[johndeighan]

$ deno --version
deno 2.2.5 (stable, release, x86_64-pc-windows-msvc)
v8 13.5.212.4-rusty
typescript 5.7.3

This bug also exists in the TypeScript compiler - I don't know if you use it directly or have your own code. In the following TypeScript file, named bug.ts:

export enum SynCat {
	INDENT,
	UNDENT
	}

export type Token = string | SynCat

function fileHandler(lTokens: Token[]): void {

	if (lTokens[0] === SynCat.INDENT) {
		lTokens.shift()
		while (lTokens[0] !== SynCat.UNDENT) {
			const tok = lTokens.pop()
		}
	}
	return
}

When I run deno test bug.ts, I get the message:

TS2367 [ERROR]: This comparison appears to be unintentional because the types 'SynCat.INDENT' and 'SynCat.UNDENT' have no overlap.
                while (lTokens[0] !== SynCat.UNDENT) {
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I think it's assuming that lTokens[0] has value SynCat.INDENT because of the preceding if check. However, inside that if statement that value of lTokens is modified by shifting off a value, and therefore, lTokens[0] has been modified. So, the comparison of lTokens[0] with SynCat.UNDENT is valid, i.e. not superfluous.

I suspect that the code will still run, but the message is wrong (unless I'm missing something). Also, it's reported as an ERROR, not a WARNING.

I did a search of TypeScript issues, and this may be related to #9998.

[marvinhagemeister]

This bug also exists in the TypeScript compiler - I don't know if you use it directly or have your own code.

Yeah we're using the TypeScript compiler directly for type checking. We don't have our own code for that.

[marvinhagemeister]

Closing as there isn't much we can do on our end. In general we don't extend the TypeScript language and leave that up to the TypeScript team. The upstream issue is here: microsoft/TypeScript#9998