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Const indexed access issue — should attempt widening to a mapped type #51156
Description
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
MartinJohns commentedon Oct 12, 2022
This is working as intended.
Kcan also be a union type, like"a" | "b". In this case your typeFuncs[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 astringor anumber- so the type you must provide must be both:string & number. But such a type can't be represented, so you end up withnever.RyanCavanaugh commentedon Oct 12, 2022
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
typescript-bot commentedon Oct 15, 2022
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.