Define type to convert array of keyed object to indexed object in typescript

Question

I want to convert an array of {key:'A',value:'B'} to an indexed object {A:{key:'A',value:'B'}} . How can I define the return type to reflect the result object ?

function toObject<T = any>(a: Iterable<T & { key }>): { [k in /* !!not like this!! */ keyof T]: T } {
  return [...a].reduce((a, v) => (a[v.key] = v, a), {}) as any;
}

// Hope there is a hint for A and B
toObject([{key:'A',value:'B'},{key:'B'}]).A

EDIT

Thanks to @Niilo Keinänen, the answer is very close, I changed to param to array, also works.

const toObject = <K extends string, T = any>(arr: Array<T & { value: K }>): { [k in K]: T }  {
  const obj: any = { };
  arr.forEach(v => obj[v.value] = v);
  return obj;
}

Answer 1

Did I understand correctly:

toObject({ key: 'first', value: 'second' })
= { first: 'second' } ?

This would seem to work:

const toObject = <K extends string, V, T extends { [P in K]: V }>(keyValuePair: { key: K, value: V }): T => {
    const obj: any = {}
    obj[keyValuePair.key] = keyValuePair.value
    return obj as T
}

Maybe you could also avoid the dirty any type somehow, not sure.