Description

Stephen Michel (smichelForHire)

opened on Oct 12, 2022

Issue body actions

Bug Report

Anders Hejlsberg (@ahejlsberg) This issue seems nearly identical to #47368, but this time for const object types which are assignable to the mapped type.

🔎 Search Terms

const indexed access; object literal indexed access; object literal widen; widen mapped type

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about indexed access.

⏯ Playground Link

Playground link with relevant code

💻 Code

type ArgMap = { a: number, b: string };
type Func<K extends keyof ArgMap> = (x: ArgMap[K]) => void;
type Funcs = { [K in keyof ArgMap]: Func<K> };

const funcs = {
    a: (x: number) => {},
    b: (x: string) => {},
};

function f1<K extends keyof ArgMap>(key: K, arg: ArgMap[K]) {
    funcs[key](arg);
}

function f2<K extends keyof ArgMap>(key: K, arg: ArgMap[K]) {
    const func = funcs[key];  // Type Funcs[K]
    func(arg);
}

function f3<K extends keyof ArgMap>(key: K, arg: ArgMap[K]) {
    const func: Func<K> = funcs[key];  // Error, Funcs[K] not assignable to Func<K>
    func(arg);
}

🙁 Actual behavior

Unexpected error in example above.

It's unexpected because throwing away type information makes this type check correctly, by widening with either const funcs: Funcs (all examples) or const func: Funcs[K] (in f2). So the compiler actually is able to confirm that indexed type is assignable to the mapped type, it just doesn't try without being explicitly prompted.

🙂 Expected behavior

No errors in example above.

Activity

1. MartinJohns commented on Oct 12, 2022

   

   Martin Johns (MartinJohns)

   on Oct 12, 2022

   Contributor

   More actions

   This is working as intended.

   K can also be a union type, like "a" | "b". In this case your type Funcs[K] resolves to (x: number) => void | (x: string) => void. When trying to call a union like this, you must provide a value that's compatible with all possible argument types, because the compiler doesn't know whether it has a function accepting a string or a number - so the type you must provide must be both: string & number. But such a type can't be represented, so you end up with never.

   f1<"a" | "b">("a", "abc")
   f1<"a" | "b">("b", 123)
2. Ryan Cavanaugh (RyanCavanaugh)

   added

   Working as IntendedThe behavior described is the intended behavior; this is not a bugThe behavior described is the intended behavior; this is not a bug

   on Oct 12, 2022
3. RyanCavanaugh commented on Oct 12, 2022

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Oct 12, 2022

   Member

   More actions

   The error is correct, though if you formulate your arguments to a specific pattern, a convenient soundness hole will be created for you. See #47109
4. typescript-bot commented on Oct 15, 2022

   

   TypeScript Bot (typescript-bot)

   on Oct 15, 2022

   Contributor

   More actions

   This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.
5. TypeScript Bot (typescript-bot)

   closed this as completedon Oct 15, 2022
6. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025

Issue actions