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Property type is not narrowed for readonly object in some contexts #19469

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@OliverJAsh

Description

@OliverJAsh

TypeScript Version: 2.4.1 and 2.6.0-rc

Code

{
    type User = { name: string };
    const logUser = (_user: User) => {};

    type State = {
        user: User | undefined;
    };

    const fn = function<Own extends {}>(p: Readonly<Own & State>) {
        const { user } = p;
        if (user !== undefined) {
            // Unexpected error: Type 'undefined' is not assignable to type 'User'
            // `user` should be narrowed to exclude `undefined` here.
            logUser(user);
        }
    };
}

//
// KNOWN WORKAROUNDS
//

{
    type User = { name: string };
    const logUser = (_user: User) => {};

    type State = {
        user: User | undefined;
    };

    const fn = function<Own extends {}>(p: Own & State) {
        const { user } = p;
        if (user !== undefined) {
            // No error when `Readonly` is dropped
            logUser(user);
        }
    };
}

{
    type User = { name: string };
    const logUser = (_user: User) => {};

    type State = {
        user: User | undefined;
    };

    const fn = function<Own extends {}>(p: Readonly<Own & State>) {
        const user: User | undefined = p.user;
        if (user !== undefined) {
            // No error when `user` is manually annotated
            logUser(user);
        }
    };
}

{
    type User = { name: string };
    const logUser = (_user: User) => {};

    type State = {
        user: User | undefined;
    };

    type Own = {};
    type P = Readonly<Own & State>
    const p = {} as any as P;

    const { user } = p;
    if (user !== undefined) {
        // No error if origin of `p` value and `Own` type is not from function
        logUser(user);
    }
}

Expected behavior:
(See comments in code.)

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