Failure to refine generic argument of type `extends boolean`

[0rvar]

TypeScript Version: 4.0, nightly

Search Terms:
Generic, refinement, refine, extends boolean

Code

function foo<TValue extends boolean>(value: TValue) {
  if (value === true) {
    const truthy: true = value
  }
}

Expected behavior:
TValue should be refined to true. At the very least, the value variable should.

Actual behavior:
Error:

Type 'TValue' is not assignable to type 'true'.
  Type 'boolean' is not assignable to type 'true'.(2322)

Playground Link:
https://www.typescriptlang.org/play?ts=4.1.0-dev.20201030#code/GYVwdgxgLglg9mABMOcA8AVAagQwDYgCmihAHlIWACYDOiARqnoTmAHwAUAbvkQFyJsvQgEpEAbwCwAKESIYwRN2GIAvOsRQATkTFTZcxBAQ0omnVAAWATwHaiaxDwKEZcgL4zP0oA

[RyanCavanaugh]

Narrowing doesn't quite work this way and it'd be difficult to make it do this without breaking other behavior. Ideally it'd be something like TValue & true but that kind of analysis only makes sense on unit values like true and false, but it's rarely useful to write generics that operate like that.

[0rvar]

Narrowing doesn't quite work this way and it'd be difficult to make it do this without breaking other behavior.

Could you elaborate?

it's rarely useful to write generics that operate like that.

Here's a slightly contrived use case:

type FullPayload = { _brand: 1 };
type LightPayload = { _brand: 2 };
type PayloadTypes = 'full' | 'light';
function callApi<TType extends PayloadTypes>(type: TType, payload: TType extends 'full' ? FullPayload : LightPayload) {
  if (type === 'full') {
    const fullPayload: FullPayload = payload; // Type error
    // serialization logic for full payload etc
  }
  // else etc
}