improper declared type cause narrow type not work when its computed type is not union. #39114

ShuiRuTian · 2020-06-17T17:32:22Z

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:

ShuiRuTian · 2020-06-17T17:51:51Z

The reason is isMatchingReferenceDiscriminant function return false at the second time. Declared type is no use here.

#38839 Add this link, for I meet similar problem in the fix.

orta · 2020-07-17T13:04:43Z

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

orta · 2020-07-17T19:17:10Z

Think I've got the bugs down, @typescript-bot run repros

typescript-bot · 2020-07-17T19:17:18Z

Heya @orta, I've started to run the code sample repros for you. Here's the link to my best guess at the log.

typescript-bot · 2022-04-13T23:29:23Z

👋 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

⚠️ Assertions:

(parameter) x: B

Historical Information

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

RyanCavanaugh added the Bug A bug in TypeScript label Jun 17, 2020

RyanCavanaugh added this to the Backlog milestone Jun 17, 2020

ShuiRuTian mentioned this issue Jun 18, 2020

'always false' field now narrowed in 'if' true-branch #39139

Closed

typescript-bot added the Has Repro This issue has compiler-backed repros: https://aka.ms/ts-repros label Jul 17, 2020

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ShuiRuTian mentioned this issue Feb 9, 2021

Type guard by deep property #38839

Closed

Dec	JAN	Apr
	22
2022	2023	2025

improper declared type cause narrow type not work when its computed type is not union. #39114

improper declared type cause narrow type not work when its computed type is not union. #39114

ShuiRuTian commented Jun 17, 2020 •

edited by orta

ShuiRuTian commented Jun 17, 2020 •

edited

orta commented Jul 17, 2020

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orta commented Jul 17, 2020

typescript-bot commented Jul 17, 2020

typescript-bot commented Apr 13, 2022 •

edited

improper declared type cause narrow type not work when its computed type is not union. #39114

improper declared type cause narrow type not work when its computed type is not union. #39114

Comments

ShuiRuTian commented Jun 17, 2020 • edited by orta

ShuiRuTian commented Jun 17, 2020 • edited

orta commented Jul 17, 2020

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This comment has been minimized.

orta commented Jul 17, 2020

typescript-bot commented Jul 17, 2020

typescript-bot commented Apr 13, 2022 • edited

ShuiRuTian commented Jun 17, 2020 •

edited by orta

ShuiRuTian commented Jun 17, 2020 •

edited

typescript-bot commented Apr 13, 2022 •

edited