How to fix this conditional type?

[short mango]

Alii#8184

Playground Link

Preview:```ts
type conditionalParams = {
/**

• run a callback fn while streaming data
  /
  onStream: (chunkValue: string) => void
  /*
• run a callback fn when streaming is done
  */
  onStreamFinished: (chunkValue: string) => void
  }

interface Re
...```

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Burrito#6903

Playground Link

Preview:```ts
type ConditionalParams = {
onStream: (chunkValue: string) => void
onStreamFinished: (chunkValue: string) => void
}

type RequiredParams = {
url: string
options: RequestInit
}

type FetcherParams = RequiredParams &
(
| {
...```

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[opal moat]

You probably want a DU.

[bleak garden]

What is a DU?

[opal moat]

Discriminated union.

The shape of FetcherParams is discriminated by stream field, in your case.

[bleak garden]

I did that!

type FetcherParams = RequiredParams["stream"] extends true
  ? RequiredParams & ConditionalParams
  : RequiredParams;```
isn't that what you mean?

[opal moat]

Yes but that wouldn't work, because RequiredParams doesn't change, RequiredParams["stream"] is always boolean, and boolean extends true is always true.

[bleak garden]

you say I need DU, and now you say it wouldn't work!

[opal moat]

Because that's not a DU

That whole thing just collapses to always the true branch.

[bleak garden]

yes, can you give an exact solution?

[opal moat]

This.