Typeguard should use conditional types

TypeScript Version: 2.9.0-dev.20180428

Search Terms:

conditional types, type guards

Code

type Pred<T, U extends T> = (v: T) => v is U;
function someFn<T, U extends T>(pred: Pred<T, U>, val: T): void {
  if (pred(val)) {
    const u: U = val;
    console.log(u);
    return;
  }
  const notU: Exclude<T, U> = val; // Type 'T' is not assignable to type 'Exclude<T, U>'.
  console.log(notU);
}

Expected behavior:

The code should compile, the predicate type-guard sets val to U in the "true" branch, and to Exclude<T, U> in the "false" branch.

Actual behavior:

In the "false" branch val is inferred to be T:

Type 'T' is not assignable to type 'Exclude<T, U>'.

Playground Link: https://bit.ly/2HAtMpF

Related Issues:

this works:

function doSth<T, U extends T>(pred: Pred<T, U>, val: T): void {
  const antiPred = (val: T): val is Exclude<T, U> => !pred(val);
  if (pred(val)) {
    const u: U = val;
    console.log(u);
    return;
  }
  if (antiPred(val)) {
    const notU: Exclude<T, U> = val;
    console.log(notU);
  }
}

I made it work by altering val: T to val: U | Exclude<T, U>, which does not feel too wrong. but my intuition says that by stating <T, U extends T>, T would be equivalent to U | Exclude<T, U>. 🤔

type Pred<T, U extends T> = (v: T) => v is U;
function someFn<T, U extends T>(pred: Pred<T, U>, val: U | Exclude<T, U>): void {
  if (pred(val)) {
    const u: U = val;
    console.log(u);
    return;
  }
  const notU: Exclude<T, U> = val;
  console.log(notU);
}

if my intuition is off here, i'll gladly close the issue.

Related to #22348

my intuition says that by stating <T, U extends T>, T would be equivalent to U | Exclude<T, U>

This isn't the case.

Ryan Cavanaugh (@RyanCavanaugh)

Ryan Cavanaugh (@RyanCavanaugh): alright i close the issue then. if you find the time, you can maybe elaborate. My naive reasoning would be: Given there is "Animal" and "Dog", stating: "Animal" would be be the same as "Dog and every Animal but Dog".

Navigation Menu

Repository navigation

Typeguard should use conditional types #23764

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Typeguard should use conditional types #23764

Description

Activity

mkulke commented on Apr 28, 2018

mkulke commented on Apr 28, 2018

mhegazy commented on Apr 30, 2018

RyanCavanaugh commented on Aug 22, 2018

mkulke commented on Aug 23, 2018

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions