Type inference regression in TypeScript v4.5

Bug Report

Sorry for the vague title, wasn’t sure how to name it otherwise

🔎 Search Terms

• type inference 4.5
• PickKeysByValue

🕗 Version & Regression Information

• This changed between versions 4.4.4 and 4.5.4

⏯ Playground Link

• Example in 4.4.4
• Same example in 4.5.4

💻 Code

type PickKeysByValue<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];

export type PickProperties<T, P> = Pick<T, PickKeysByValue<T, P>>;

const obj = {
  a: 'a',
  b: 15,
};

function acceptOnlyString(str: string): void {}

function fn<T extends object, K extends PickKeysByValue<T, string>>(item: T, key: K) {
  acceptOnlyString(item[key]); // Argument of type 'T[K]' is not assignable to parameter of type 'string'.
}

🙁 Actual behavior

Line acceptOnlyString(item[key]); throws an Error Argument of type 'T[K]' is not assignable to parameter of type 'string'.

🙂 Expected behavior

The line should pass - key is limited to only keys for which item[key] is string

[RyanCavanaugh]

This code only worked in 4.4 because the type wasn’t being checked at all:

type PickKeysByValue<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];

export type PickProperties<T, P> = Pick<T, PickKeysByValue<T, P>>;

const obj = {
  a: 'a',
  b: 15,
};

function acceptOnlyString(str: string): void {}
function acceptOnlyNumber(num: number): void {}

function fn<T extends object, K extends PickKeysByValue<T, string>>(item: T, key: K) {
  // OK
  acceptOnlyString(item[key]);
  // Also OK!
  acceptOnlyNumber(item[key]);
}

We don’t yet have logic that can figure out that K extends PickKeysByValue means item[key] must be a subtype of string. I’m going to make a follow-up proposal for this since it’s an oft-requested feature

[kaya3]

A workaround is to declare the type of item as T & Record<K, string>. This fixes the type error in the function, and usually won’t cause an error at the call-site.

function fn<T extends object, K extends PickKeysByValue<T, string>>(item: T & Record<K, string>, key: K) {
  acceptOnlyString(item[key]);
}

Playground Link