Type infomation is missed when recursively use generic type & interface

[xcatliu]

TypeScript Version: 2.8.3

Search Terms: type keyof extra

Code

type Template<T> = {
    [K in keyof T]: Template<T[K]>;
} & {
    __name?: string;
}

const template: Template<{
    foo: number;
    bar: {
        baz: string;
    }
}> = {
    foo: 1,
    __name: 'n',
    bar: {
        baz: 'b',
        not_exist_key: true
    }
};

Expected behavior: Should report error: 'not_exist_key' does not exist in type 'Template<{ baz: string; }>'

Actual behavior: No errors

Playground Link: playground

Related Issues:

[jack-williams]

Edit: I think this is probably related to #13813

I think this is do to with excess property checking. Here is a smaller example:

type A = { x: { y: string } };
let x: A = { x: { y: "hello", z: true } }; // Good - Error. z is excess

type B = A & { extra: number; }
let y: B = { x: { y: "hello", z: true }, extra: 42 }; // Bad - No error (z is still excess).

I believe the issue is that excess property checking happens at one level at a time. After the first level TypeScript sees the intersection type and no longer marks the source type as a fresh object literal, which then disables excess property checking for the second level (properties y and z).

[xcatliu]

@jack-williams Thanks for your reply. The smaller example make this issue clearer.
Another question is why can't I add an extra key to mapped types like this:

type Template<T> = {
    [K in keyof T]: Template<T[K]>;
    foo?: string;
}
// Cannot find name 'foo'.
// '}' expected.

[mhegazy]

Duplicate of #13813. thanks @jack-williams!

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.