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Cannot use mapped types to create new mapped types #48246
Description
Bug Report
Cannot use mapped types to create new mapped types. I have no idea why this would happen because the types are computed correctly.
🔎 Search Terms
- mapped types
🕗 Version & Regression Information
This changed between version 4.5.5 and 4.6.2.
⏯ Playground Link
Playground link with relevant code
💻 Code
function makeCompleteLookupMapping<T extends ReadonlyArray<any>, Attr extends keyof T[number]>(
ops: T,
attr: Attr,
): {
[Item in T[number]as Item[Attr]]: Item;
} {
return Object.fromEntries(
ops.map(op => [op[attr], op] as const),
);
}
const ALL_BARS = [
{ name: 'a'},
{name: 'b'},
] as const;
const BAR_LOOKUP = makeCompleteLookupMapping(ALL_BARS, 'name')
type BarLookup = typeof BAR_LOOKUP;
type Baz = { [K in keyof BarLookup]: BarLookup[K]['name']; };🙁 Actual behavior
I cannot use name to index BarLookup[K] where K is a keyof BarLookup.
🙂 Expected behavior
I expect type Baz to be {a: 'a', b:'b'}.
Activity
Andarist commentedon Mar 14, 2022
This looks strange to me too - especially given that this worked before. As a temporary workaround you can use a
Prophelper, like here.What I've also noticed is that when indexing this
BarLookup[K][/*HERE*/]we get string methods in the autocomplete as if would think that the type ofBarLookup[K]is a string or somethingRyanCavanaugh commentedon Mar 14, 2022
Bisected to #47370
ahejlsberg commentedon Mar 15, 2022
This and #48157 are caused by the same core issue. We need to exclude mapped types with optionality modifiers and/or
asclauses from the new constraint logic introduced in #47370.