TS 4.2 Promise.all can't deal with array + spread

[bfricka]

Bug Report

🔎 Search Terms

• no overload matches call created:>=2021-01-01
• promise created:>=2021-01-01
• iterable created:>=2021-01-01

🕗 Version & Regression Information

• 4.2.2
• 4.2.3
• Still reproduces in nightly 4.3.0-dev.20210308
• Works fine in 4.1 and below

• This changed between versions 4.1.5 and 4.2

⏯ Playground Link

Playground link with relevant code

💻 Code

const arrFoo = async () => ['foo']
const foo = async () => 'foo'
const bar = async () => 'bar'

const test = async () => {
    const mapToBarPromises = [1, 2, 3].map(bar)
    
    const onlyBars = await Promise.all(mapToBarPromises)
    const onlyBarsSpread = await Promise.all([...mapToBarPromises])
    const mixed = await Promise.all([foo(), bar()])
    const mixedWithArr = await Promise.all([arrFoo(), bar()])
    const mixedWithSpread = await Promise.all([foo(), ...mapToBarPromises])
    // Fail when mixing spread with other arrays
    const mixedArrAndSpread = await Promise.all([arrFoo(), ...mapToBarPromises])
}

🙁 Actual behavior

TypeScript 4.2 Promise.all can't deal with a Promise<T1[]> plus a spread Promise<T2>

🙂 Expected behavior

This shouldn't be an error and in previous versions, it wasn't. Since it's very common for something async to return a list it makes it painful to do something like:

const [teams, ...players] = await Promise.all([getTeams(), ...regions.map(getPlayers)])

Sorry, I couldn't think of a less contrived example. More likely in code I see, the spread is a list of side effects updates, where we don't actually care about the result:

await Promise.all([
  updateUsers(),
  ...teams.map(updateTeam),
])

I can do Promise.all<any>(), but if we actually want the values, we have to do some elaborate typing that we shouldn't have to.