Error using indexed access types with mapped types

[irudoy]

Bug Report

🔎 Search Terms

indexed access types, mapped types

🕗 Version & Regression Information

• This changed between versions 4.6.4 and 4.7.0

⏯ Playground Link

v4.8.0-dev.20220525 ❌

v4.7.0-beta ❌

v4.6.4 ✅

v4.5.5 ❌

💻 Code

type Types = {
  first: { a1: true };
  second: { a2: true };
  third: { a3: true };
}

class Test {
  entries: { [T in keyof Types]?: Types[T][] }

  constructor() {
    this.entries = {};
  }

  addEntry<T extends keyof Types>(name: T, entry: Types[T]) {
    if (!this.entries[name]) {
      this.entries[name] = [];
    }

    this.entries[name]?.push(entry); // error
  }
}

🙁 Actual behavior

Got the following error:

Argument of type '{ a1: true; } | { a2: true; } | { a3: true; }' is not assignable to parameter of type '{ a1: true; } & { a2: true; } & { a3: true; }'.
  Type '{ a1: true; }' is not assignable to type '{ a1: true; } & { a2: true; } & { a3: true; }'.
    Property 'a2' is missing in type '{ a1: true; }' but required in type '{ a2: true; }'

🙂 Expected behavior

No error. Or maybe I missed something, and this was intentional.

Seems like it was fixed in #47109 and broken in v4.7.

[jcalz]

Looks like something's going on when the mapped type is optional. This happened between 4.7.0-dev.20220302 and 4.7.0-dev.20220329. It would definitely be nice if this didn't stay broken in 4.7.x for long.

[ahejlsberg]

Yeah, this is a bug. We're being a bit too conservative in the isMappedTypeGenericIndexedAccess function. It disallows any ? modifiers when really it should only disallow -?. Easy fix, I'll have a PR up soon.