Combination of intersection type, mapped type and generic type seem to break type checks for nested properties

[SevInf]

Bug Report

🔎 Search Terms

mapped types, generic bound, nested objects

🕗 Version & Regression Information

We've detected this on 4.7.4. Checking previous versions on playgrounds, last one where it worked correctly is 3.5.1.
It is also reproducible on current nightly build.

• This changed between versions 3.5.1 and 3.6.3

⏯ Playground Link

Playground link with relevant code

💻 Code

type BaseType = {
  select?: { id?: boolean} ,

  where: {},
}

type FullType = BaseType & {
  anything?: boolean
}

type MappedType<T> = {
  [key in keyof T]:  T[key]
}

declare function genericFunction<T extends FullType>(args: MappedType<T>): void

genericFunction({
  where: {},
  select: {
    id: true,
    // this should not pass
    shouldErrorHere: 'lalalala'
  }
})

🙁 Actual behavior

The code passes typechecks

🙂 Expected behavior

shouldErrorHere property should not be allowed in that position, since it's not defined on typeof BaseType['select']. Doing any of the following causes expected error to appear:

1. Removing intersection type and making FullType a plain alias for BaseType
2. Making where property optional in BaseType definition
3. Removing anything property from FullType
4. Adding any non-optional property to the intersection
5. Omitting where property in genericFunction call
6. Passing incorrect anything value during the call
7. Changing FullType to interface FullType extends BaseType

[ahejlsberg]

I think this is working as intended and not a bug. In the call to genericFunction we infer the following type argument for T:

{
    where: {};
    select: {
        id: true;
        shouldErrorHere: string;
    };
}

This means that shouldErrorHere isn't an excess property, it's part of the type we've inferred. Now, separate from this we also validate that the inferred type is assignable to the FullType constraint, but this check doesn't perform excess property checking. You can observe the difference by changing the type of args from MappedType<T> to MappedType<FullType>. In the latter case you will indeed get an excess property error.

[SevInf]

@ahejlsberg Thank you for the explanation!

However, if that's the case, wouldn't the same also hold true for all of the other case I listed at the end of my post? For example, this one. The only difference with the original one is that here, where property of BaseType is not required. In my understanding, inferred type for T should not change in this case, however, excess property error now appears.

[ahejlsberg]

Hmm, yeah, something is definitely suspicious there.