A generic in a custom type guard is not considered for the return type

[mckravchyk]

Bug Report

🔎 Search Terms

generic custom type guard

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

Playground link with relevant code

💻 Code

type FooId  = `foo_${string}`
type BarId = `bar_${string}`
type FooBarId = FooId | BarId;

interface Foo {
    id: FooId
    type: 'foo'
}

interface Bar {
    id: BarId
    type: 'bar'
}

type FooBar = Foo | Bar;

function isFoo(id: FooBarId): id is FooId { }

function getFoo(id: FooId): Foo { }

function getBar(id: BarId): Bar { }

type FooBarFromId<T extends FooBarId> =
  T extends FooId ? Foo :
  T extends BarId ? Bar :
  FooBar;

function getFooBar<T extends FooBarId>(id: T): FooBarFromId<T> {
  // Type 'Foo' is not assignable to type 'FooBarFromId<T>'
  return isFoo(id) ? getFoo(id) : getBar(id);
}

// I'm using multiple declarations as a workaround

function getFooBar(id: FooId): Foo;
function getFooBar(id: BarId): Bar;
function getFooBar(id: FooBarId): FooBar;
function getFooBar(id: FooBarId): FooBar {
  return isFoo(id) ? getFoo(id) : getBar(id);
}

🙁 Actual behavior

I have a function that accepts a generic constrained argument and has a return type that depends on the generic. The return type of the implementation is not compatible with the return type in the definition.

🙂 Expected behavior

There should be no error in the return statement. FooBarFromId type and the return statement implementations are compatible, it's like the compiler is not considering the generic in the equation.

[MartinJohns]

It's a design limitation. Resolving of conditional types is deferred when unbound type arguments are involved. In order to resolve FoBarFromId<T> the compiler needs to know what type T is, but within the function it can't know what T is because it's externally supplied.

[fatcerberus]

See also #33912.

[mckravchyk]

I see. Thanks for explaining this. I hope that some day it will be possible.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.