Missing narrowing of intersections involving type parameter and primitive types

[ahejlsberg]

🕗 Version & Regression Information

• This is the behavior in every version I tried.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABMAjAHgCoD4AUAPALkQ0QDJEBnKAJxjAHNEAfYsxcAEwFNg6uOAlIgDeAWABQiKYhjBE+IWMnSVeANxSA9JsQBlABZwQAGw6IwAQ2rU4Ad0RQ4rclVoMJKgL4Tv4iaEhYBGQAJkxcQmdKGjpGFhJyTh4+QREPaVl5PEQAQgBePPYwbl4wfkV0lSl1LR0DI1NzKxt7RyjXWMrEXxUuYwouNOUqxBrEbT1DEzNLazsHJwSikpSu319-cGh4JGAAZnD8IiWOhmYosBAAWwAjLmoK4Zk5HCgATwAHLjg5bILCgBEp3oAMeI2qGnGdSmjVmLQW7Ri7iePWkfQGQ3Bo0hE3q0yac1aizYl1u9zWPgkm0CO2QABZDpElihzksQmDpBRbDAoBB9FkOSMIBYMSgiDUbtQuBYANY4nTMroqYUYkLijSS6Vy2pREJK6QlCwmKDqnV42HNeZtMoAN3JKMpfnEAW2wWAAFZMIguHgoFxihRom44uZrndqBFjiSw-dBYgIAgqA4WYU-gVEAD9H1jHAAfLJg0zHdEBYkPcbNRHUA

💻 Code

function f1<T>(x: T & string | T & undefined) {
    if (x) {
        x;  // Should narrow to T & string
    }
}

function f2<T>(x: T & string | T & undefined) {
    if (x !== undefined) {
        x;  // Should narrow to T & string
    }
    else {
        x;  // Should narrow to T & undefined
    }
}

function f3<T>(x: T & string | T & number) {
    if (typeof x === "string") {
        x;  // Should narrow to T & string
    }
    else {
        x;  // Should narrow to T & number
    }
}

function f4<T>(x: T & 1 | T & 2) {
    switch (x) {
        case 1: x; break;  // T & 1
        case 2: x; break;  // T & 2
        default: x;  // Should narrow to never
    }
}

function f5<T extends string | number>(x: T & number) {
    const t1 = x === "hello";  // Should be an error
}

🙁 Actual behavior

No narrowing or errors where expected.

🙂 Expected behavior

Narrowing and errors as indicated above.