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.