parameter initializer wrongly references variable declared in function body

[ajafff]

TypeScript Version: 3.2.0-dev.20181101

Search Terms:

Code

let baz = 2;
function foo(bar = () => baz) {
    var baz = 1;
    return bar();
}

Expected behavior:
No error.
baz in the initializer references the outer let baz = 2.
let baz = 1 is marked as unused.

Actual behavior:
Error Initializer of parameter 'bar' cannot reference identifier 'baz' declared after it.

At runtime: calling foo() returns 2, which shows that declarations in the function body are not in scope of parameter initializers.

Note that other parameters are in scope and can be used inside other parameters' initializer.

Playground Link: https://agentcooper.github.io/typescript-play/#code/DYUwLgBARghgXhAvBATAbgFADMCuA7AYzAEsB7PCLU0gClgCckIaBKJAPmnjYG8MIBEAG4xGsBMgCMmQRHrgc9Cg1aYAvkA

Related Issues:
#22769
/cc @Kingwl maybe you want to give this a shot as well (I hope this doesn't cause as much trouble as the last one 😆)

[Kingwl]

🤷🏻‍♂️ holy shit
I'will take a look soon

[ajafff]

Continuing discussion from #28501 (comment)

@weswigham @Kingwl

I'm pretty sure that error reflects the behavior of our actual down level emit rather than the spec.

That's correct, but we already had that discussion in #22769. I'd argue that the emit should be changed to follow the spec. However, in #22769 (comment) there's the decision to change the lookup for ES6 and emit an error for ES5 and below (which means no change for that target).

That means it's still a bug if target is ES6 or above and still needs fixing.

[Kingwl]

ahhhhhh, I'll reopen that

[sandersn]

This is fixed in 3.8.