Protected field with same name in two classes messing up type constraint when narrowing array.

[Jym77]

Bug Report

(I'm not even sure how to properly name that issue 🙃 )

🔎 Search Terms

typescript inheritance protected intersection

🕗 Version & Regression Information

Seen it recently in 4.6.2. Update to 4.7.3 without changing the behaviour (and 4.8.0 nightly in the playground). I don't know if it was here before.

• I was unable to test this on prior versions because this triggered with unrelated change elsewhere in my code

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo {
  protected foo = 0;
}

class Bar {
  protected foo = 0;
}

type Nothing<V extends Foo> = void;

type Broken<V extends Array<Foo | Bar>> = {
  readonly [P in keyof V]: V[P] extends Foo ? Nothing<V[P]> : never;
};

🙁 Actual behavior

(error located at the V[P] in Nothing<V[P]>)

 Type 'Foo & V[P]' does not satisfy the constraint 'Foo'.
  Property 'foo' is missing in type 'Foo & V[P]' but required in type 'Foo'.

Changing foo from protected to public in either Foo or Bar makes the error disappear.

Removing (or renaming) protected foo = 0 from either Foo or Bar makes the error disappear.

Replacing the Array<Foo | Bar> in Broken by Array<Foo> makes the error disappear.

🙂 Expected behavior

Since V[P] is already in a type guard that is the same as the constraint for Nothing, this should not generate any typing error.

[RyanCavanaugh]

Workaround

type Broken<V extends Array<Foo | Bar>> = {
  readonly [P in keyof V]: V[P] extends (infer T extends Foo) ? Nothing<T> : never;
};

[Jym77]

Thanks. Workaround works fine in the real code too 😄

[Jym77]

It seems that the workaround breaks api-extractor 😢

[andrewbranch]

@typescript-bot bisect good v4.6.4 bad v4.7.4

[typescript-bot]

The change between v4.6.4 and v4.7.4 occurred at 787bb9ddb60368f84b1ed1b0971aa2bf79fdec77.

[andrewbranch]

Hey this is the second bisect today that blames #48837 😅

[typescript-bot]

:wave: Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

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Issue body code block by @Jym77

:x: Failed: -

• Type 'Foo & V[P]' does not satisfy the constraint 'Foo'. Property 'foo' is missing in type 'Foo & V[P]' but required in type 'Foo'.

Historical Information

Version	Reproduction Outputs
4.7.2	:x: Failed: - Type 'Foo & V[P]' does not satisfy the constraint 'Foo'. Property 'foo' is missing in type 'Foo & V[P]' but required in type 'Foo'.
4.3.2, 4.4.2, 4.5.2, 4.6.2	:+1: Compiled