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Object destructuring type doesn't narrow parameter B when parameter A is narrowed #44169

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Closed
Object destructuring type doesn't narrow parameter B when parameter A is narrowed#44169
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In DiscussionNot yet reached consensusSuggestionAn idea for TypeScript

Description

@icecream17
Steven Nguyen (icecream17)
opened on May 19, 2021

Bug Report

🔎 Search Terms

destructuring
narrowing
is not assignable to type

🕗 Version & Regression Information

(todo)

  • This is a crash
  • This changed between versions ______ and _______
  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
  • I was unable to test this on prior versions because _______

💻 Code

This works

type Dastructure = 
   {b: false, c: null} |
   {b: true, c: string}

class Example {
   prop?: string;
   constructor (options: Dastructure) {
      if (options.b) {
         this.prop = options.c
      }
   }
}

This gets an error

type Dastructure = 
   {b: false, c: null} |
   {b: true, c: string}

class Example {
   prop?: string;
   constructor ({b, c}: Dastructure) {
      if (b) {
         this.prop = c
//       ^^^^^^^^^
// Type 'string | null' is not assignable to type 'string | undefined'.
//   Type 'null' is not assignable to type 'string | undefined'.
      }
   }
}

🙁 Actual behavior

error

🙂 Expected behavior

not error?

I think it's because in the second code the context of {b, c}: A isn't considered when b is narrowed

Activity

  1. Steven Nguyen (icecream17)
    changed the title [-]Object destructuring for parameters don't narrow[/-] [+]Object destructuring type doesn't narrow parameter B when parameter A is narrowed[/+] on May 19, 2021
  2. Ryan Cavanaugh (RyanCavanaugh)
    added
    In DiscussionNot yet reached consensus
    SuggestionAn idea for TypeScript
    on May 19, 2021

  3. RyanCavanaugh commented on May 19, 2021

    @RyanCavanaugh
    Ryan Cavanaugh (RyanCavanaugh)
    on May 19, 2021
    Member

    This is a duplicate but I want to reraise the idea of CFA-linking variables sourced from the same destructuring expression

  4. dhythm commented on May 21, 2021

    @dhythm
    Yuta Okada (dhythm)
    on May 21, 2021

    I agree with this suggestion.
    Type inference doesn't work well when I use destructuring.
    I'm happy if this issue is solved. 🙏

    image

  5. remcovaes commented on Aug 9, 2021

    @remcovaes
    remcovaes
    on Aug 9, 2021

    Is there any update on this? I am running into the exact same issue...

    type F = ({ foo, bar }: { foo: number, bar?: never } | { foo?: never, bar: true } ) => number;

const f: F = ({ foo, bar }) => {
	if (bar === true) {
		return 2;
	}

	return foo;
};

    https://www.typescriptlang.org/play?ts=4.3.5#code/C4TwDgpgBAYlC8UAUBvKAzA9pgNFARgIYBOUAvgFxRpaZUB2ArgLb4TF5HED8DEAbu3JQAPtQzZeUegPacSVYMUbQyUAJQIAfNJZtiAbgBQRgMaZ6AZ2AYqcRKgm4CJcpvg6URgJABLdMhcCPCISiqaXt7exBDAjMT0UABMxt5kJtGx8Ym0xmTGQA

  6. icecream17 commented on Nov 17, 2021

    @icecream17
    Steven Nguyen (icecream17)
    on Nov 17, 2021
    Author

    This seems to be fixed by #46266
    https://www.typescriptlang.org/play?ts=4.6.0-dev.20211117#code/C4TwDgpgBAIghgZ2AJwK4GNiudAvFAWACgpSBvAIwC4oAzOAGwQgBop0aA7VBhgXygAfYqSiUaKVK3Y0kyAJacA5n2LF0DRAigBRAB5wAtmAbQyI0mGQB7MAH5ZKRUoDcF9tc5yMwa8igAFJRs6Hw08N6Y2BAAlGLuovK0gRRx5iSimVDAABbyCAB0VrZQ+OgJpKoZUFV8QA

  7. Steven Nguyen (icecream17)
    closed this as completedon Nov 17, 2021
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