Cannot use mapped types to create new mapped types

[libre-man]

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'}.

[Andarist]

This looks strange to me too - especially given that this worked before. As a temporary workaround you can use a Prop helper, 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 of BarLookup[K] is a string or something

[RyanCavanaugh]

Bisected to #47370

[ahejlsberg]

This and #48157 are caused by the same core issue. We need to exclude mapped types with optionality modifiers and/or as clauses from the new constraint logic introduced in #47370.