Multi-line top-level `await` causes duplicate declaration error

🔎 Search Terms

typescript top-level await export class "duplicate identifier"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about await, identifiers, modules

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240827#code/AYKAFAtg9gJgrgGwKYgJBgGZwHYGMAEYSAHgA5QBOALvgEQCWAbACwB05pu2VtAlIaQCGFQRHxNm-MBSQBnRDXoBmAEy8Q+TfgRRcghKwDmSGgAYNWieyiduFzcpWsA7iNIB9Cb3WgA9L-wAN3pBfAALKipSWQAuf2ckACNBWVkkCESEAE8jeiowuETWeihfZ0FEql8YdNLyqhVy2QhfEFwobFkaJrEAXkJBcry0AHUkgEFU9Myc+k6qQW4QqiQAZSoZUTnDMDRUDBNcMLBaGEEFmMFSUgR6PSoS7DKUiABuZLSWABpxgHEAMQAXokAIoAUXGkPGACFoYZ-oZxgANZjQjDOACyAGFxiDxgAJAAihjBAGlBAAVACapgAMthoYFcL8AHJwGBQrGGQywxGI-E4LHOcak6G-ACsYUSIwAqjjnBDIb1enwvnsAN4AX28rxAbQ6XXE80WuCQ+H6PWKxrwSF1JHI1Hw7Xm+A47RwNH6cy6JqQrHtlCosgA2gByKxu7ihgC6+BShECMXwiXohjmVH4vQAfPhsHAMkgKLqQAHHbgEClZPhoXlViZ8Or7PgAMSGChQZxgXhakCavX+fDQeDIfA4doQCBIbjuSfuKhQdwYejEBuaoA

💻 Code

`
(module
	(func (export "i64.popcnt") (param i64) (result i32)
    local.get 0
    i64.popcnt
    i32.wrap_i64))
`
// via https://webassembly.github.io/wabt/demo/wat2wasm/
const wasm = (await
	WebAssembly.instantiateStreaming(
		fetch("data:application/wasm;base64,AGFzbQEAAAABBgFgAX4BfwMCAQAHDgEKaTY0LnBvcGNudAAACggBBgAgAHunCwAKBG5hbWUCAwEAAA=="),
		{}));

const instance = wasm.instance;
export const popcount = instance.exports['i64.popcnt'] as (v: bigint) => number;

export class BitSet {
    #grow(){}
}

// module uncomment_me_to_fix {}

🙁 Actual behavior

Line 17 (export class BitSet) reports an error: Duplicate identifier 'BitSet'.(2300)

As far as I can see there's no duplicate identifier; suppressing that line with a @ts-expect-error produces a strange result that the exported BitSet is unusable when imported into a different module, whereupon Typescript reports:

Type 'import("./lib/bitset").BitSet' is not assignable to type 'import("./lib/bitset").BitSet'. Two different types with this name exist, but they are unrelated. Property '#grow' in type 'BitSet' refers to a different member that cannot be accessed from within type 'BitSet'.ts(2719)

Uncommenting the empty module definition at the bottom of the file appears to collapse the superimposed wave function of BitSet, and renders the type singular again.

🙂 Expected behavior

To see neither error, whether or not there's a module in the same file.

Additional information about the issue

Hopefully, it's clear enough from the example what I'm hoping to do here with the top-level await; I deleted all the method bodies for brevity, so you'll have to trust me when I say popcount is a very useful primitive to have.

I did spend a fair bit of time looking for suggestions on whether I'm holding the tool wrong with the await ....; export class ... sequencing, but as far as I can tell that is how it's intended to be used. The fact that an unrelated (to my eye, anyway) expression later on in the file changes the compiler's report is what finally moved me to file an issue against tsc here.

Thanks for all your work on Typescript!