tsc 3.6.3: `Promise.all` breaks with some union type

[paul-marechal]

TypeScript Version: 3.6.3

Search Terms: promise all

Code

type MaybePromise<T> = T | Promise<T> | PromiseLike<T>

function MaybePromise<T>(value: T): MaybePromise<T> {
 return Promise.resolve(value)
}

async function main() {
 let aaa: number
 let bbb: string
 [
 aaa,
 bbb,
 ] = await Promise.all([
 MaybePromise(1),
 MaybePromise('2'),
 MaybePromise(true),
 ])
}

tsconfig:

{
 "compilerOptions":{
 "strict": true,
 "target": "es5",
 "module": "commonjs",
 "lib": [
 "es6"
 ]
 }

Expected behavior:

Code should compile without errors.

Actual behavior:

Error:

test.ts:11:9 - error TS2322: Type 'number | Promise<1>' is not assignable to type 'number'.
 Type 'Promise<1>' is not assignable to type 'number'.
test.ts:12:9 - error TS2322: Type 'string | Promise<"2">' is not assignable to type 'string'.
 Type 'Promise<"2">' is not assignable to type 'string'.

Code compiles well with 3.5.3, but not 3.6.3 nor 3.7.0-dev.20190922.

Playground Link:

Weirdly enough, on the playground the return value of Promise.all is not a tuple but a list.

https://www.typescriptlang.org/play/#code/C4TwDgpgBAsghiARhACgJwPYFsCWBnCAHgBUA+KAXimKgB8p1t8iy6HNcCAZHAaxdIAoQQDMArgDsAxsBwYJsBMkacBACgBucADZiIALmoBKQ-CSoOzEuQDegqA6hoIwMWgUrmAOmd4M2jQhNHT0jQQBfYTg8EGkocWlZeSgsOBwJNSMoO0cobRcoOCLDCTEsZDR7R3zgKER6wzxgNHSAcyqHAG0O3MKigBoe3PrEQdyAXUpCgHc02s8CLx1tNW7exzNlSwI1AEYjMfXFcwWggHIAJjODoY2lCyYd5tDDh3GwyKA

Related Issues:

• Maybe related? new readonly modifiers on Promise.all break lyricist in DT #33559

[paul-marechal]

Encountered this issue while updating tsc, will stay on 3.5.3 until this is resolved :)

[sandersn]

#33559 is only in 3.7, so is probably not related.