3.7-beta regression: Promise.all wrongly adds `undefined` type

[Swatinem]

TypeScript Version: 3.7.0-beta

Search Terms:

promise all

Code

interface A {
  a: string
}
interface B {
  b: string
}
async function main(): Promise<A> {
  const [a, b] = await Promise.all([
    Promise.resolve({a: "a"} as A),
    Promise.resolve({b: "b"} as B | undefined), // <- the `| undefined` here is the problem
  ]);
  return a; // <- a has type `A | undefined`, but should be `A`
}

Expected behavior:

This is a regression from 3.6.3, where the return tuple of Promise.all is correctly inferred.

Actual behavior:

A | undefined bound of a different tuple member also adds a | undefined bound on a different unrelated return value of Promise.all.

Playground Link:

https://www.typescriptlang.org/play/?ts=3.7-Beta#code/JYOwLgpgTgZghgYwgAgILIN4ChnLgLmQGcwpQBzLAXy1ElkRQCFMdkAjQkskSmuIgE8QCZDACuIsMAD2IZAFs4oABQBKQgAUoMhcCIQAPKgB8rXAjklkAbTgAaDgF1kAXjwB3ZWGTbd+iAA6OAAbEJUbNlw-PQNAqAgiGRCANwgVDAJkACI4bKo8IjQ1eyjfHVighKTU9IxOHPZ8wuQWAB9kSQATCBhQCC6S5AB6YeRDAFpkMAALFAADDu7e-q755DmE5H1pueQABx12EIgFNic1AG42BLBxKHk4S5GxybwNgWnBfYX0JZAen0QAN5o52OIfEQZjJxCEuhxfvNqEA

Related Issues:

Maybe #33707 ?

[jack-williams]

I think it's caused by #33057 which is merged; #33707 is not merged so will not be in the beta.

[Swatinem]

Hm, but maybe #33707 will resolve this regression? That’s what I thought, haven’t checked though.

[jack-williams]

Reduced repo:

interface A {
  a: string
}
interface B {
  b: string
}

interface Foo {
  all<T1, T2>(values:  readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>]): Promise<[T1, T2]>;
}

declare const Foo: Foo;

function main() {
  let aValue: A;
  Foo.all([Promise.resolve({a: "a"} as A), Promise.resolve({b: "b"} as B | undefined)]).then(([a,b]) => aValue = a);
}

Error on assignment to aValue; removing readonly removes the error.

[jack-williams]

Yes, the typings in #33707 fix the smaller repro above.

[eamodio]

Here is another case of this: #34554

[buu700]

sed -i 's|: readonly|:|g' node_modules/typescript/lib/lib.es2015.promise.d.ts seems to be an effective temporary workaround until this is fixed.