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cannot enforce type shape when assigned to a conjunction #28642

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cannot enforce type shape when assigned to a conjunction#28642
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In DiscussionNot yet reached consensusSuggestionAn idea for TypeScript

Description

@sledorze
Stéphane Le Dorze (sledorze)
opened on Nov 21, 2018

TypeScript Version: 3.1.6, 3.2.0-rc, 3.3.0-dev.20181121

Search Terms:
conjunction assignable wrong type

Code

declare const a: { a: string; b: number }
declare const b: { a: string } & { b: number }
declare function f(x: { [k: string]: string }): void
f(a) // error
f(b) // NO error

Expected behavior:
Errors on both f(a) and f(b)

Actual behavior:
Errors only on f(a)

Playground Link: http://www.typescriptlang.org/play/#src=declare%20const%20a%3A%20%7B%20a%3A%20string%3B%20b%3A%20number%20%7D%0D%0Adeclare%20const%20b%3A%20%7B%20a%3A%20string%20%7D%20%26%20%7B%20b%3A%20number%20%7D%0D%0Adeclare%20function%20f(x%3A%20%7B%20%5Bk%3A%20string%5D%3A%20string%20%7D)%3A%20void%0D%0Af(a)%20%2F%2F%20error%0D%0Af(b)%20%2F%2F%20NO%20error

Related Issues: gcanti/io-ts#234 (comment)
gcanti/fp-ts-routing#37

Activity

  1. Stéphane Le Dorze (sledorze)
    changed the title [-]conjunction assignable wrong type[/-] [+]cannot enforce type shape when assigned to a conjunction[/+] on Nov 21, 2018

  2. sledorze commented on Nov 21, 2018

    @sledorze
    Stéphane Le Dorze (sledorze)
    on Nov 21, 2018
    Author

    I'm guessing this type relaxation is required in certain scenario (with named members), however, with an indexed type on string (covering both 'a' and 'b'), I would rather expect it to fail.

  3. jack-williams commented on Nov 21, 2018

    @jack-williams
    Jack Williams (jack-williams)
    on Nov 21, 2018
    Collaborator

    From the spec: S is a subtype of T if S is an intersection type and at least one constituent type of S is a subtype of T. (same for assignability).

    • T & U is assignable to X if T is assignable to X.
    • { a: string } is assignable to { [k: string]: string }

    String indexers are unsound like this when properties are 'forgotten'. Another example:

    • { a : string; b: number } is assignable to { a: string }
    • { a: string } is assignable to { [k: string]: string }
    const a: { a: string; b: number } = { a: 'foo', b: 42 };
const b: { a: string } = a
const c: { [k: string]: string } = c;
c.b = "not a number"
const shouldBeNumber: number = a.b;

  4. sledorze commented on Nov 21, 2018

    @sledorze
    Stéphane Le Dorze (sledorze)
    on Nov 21, 2018
    Author

    Jack Williams (@jack-williams) indeed.
    The amount of consequences of the unsoundness is frightening.

  5. MartinJohns commented on Nov 21, 2018

    @MartinJohns
    Martin Johns (MartinJohns)
    on Nov 21, 2018
    Contributor

    Jack Williams (@jack-williams) Quoting the spec is not advised, as the spec was last updated around 3 years ago.

  6. Wesley Wigham (weswigham)
    added
    SuggestionAn idea for TypeScript
    In DiscussionNot yet reached consensus
    on Nov 26, 2018

  7. jakebailey commented on Jul 11, 2023

    @jakebailey
    Jake Bailey (jakebailey)
    on Jul 11, 2023
    Member

    This was fixed by #35143, whose description contains effectively this issue's repro

  8. Jake Bailey (jakebailey)
    closed this as completedon Jul 11, 2023
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