improper declared type cause narrow type not work when its computed type is not union.

TypeScript Version: 4.0

Search Terms:

Code

interface A {
    type:'A'
}

interface B {
    type:'B'
}

function assert(x:any) :asserts x is A|B{

}

function foo(x: any): any {
    x;  // any
    assert(x);
    x;  // A | B
    if (x.type === 'A') {
        return x;
    }
    x;  // B
    if (x.type === 'B') {
        return x;
    }
    x;  // B, but should be never.
//  ^?
}

Expected behavior: the last x should be never.

Actual behavior: still B.

Playground Link: https://www.typescriptlang.org/play/index.html?ts=4.0.0-dev.20200615&ssl=1&ssc=1&pln=25&pc=2#code/JYOwLgpgTgZghgYwgAgIIG8BQyfLATwAcIAuAclTMwF9NNRJZEUAhLXPI0slq2zGAFcQCMMAD2IZHADOM6GAAUADxJwQ+AJTI1chTOTLkwA6gA+bOvyEixk5DHHiVJaRs2v1+ZO1zKA3DgA9EFu+Ni4svJQSsqa-hE4AcGhqMhmyCyJxjDIKgB0BMTIALxlyBRk2r4cOFAQYIJQUgHZtBzJyCGZ2cC5BUUoZSUVvNXZHPWNzYYJHO1+gV2hWdRAA

Related Issues:

[orta]

( I sneakily edited the issue body to be a twoslash compiler repro to test the bot )

[typescript-bot]

👋 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.

---

Issue body code block by @ShuiRuTian

:warning: Assertions:

• (parameter) x: B

Historical Information

Version	Reproduction Outputs
4.2.2, 4.3.2, 4.4.2, 4.5.2, 4.6.2	:warning: Assertions: (parameter) x: B

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