microsoft / microsoft/TypeScript

Generics in union should be inferred as `never` when possible

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Description

### 🔎 Search Terms

generics, inference, union, never

### 🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about union and never.

### ⏯ Playground Link

https://www.typescriptlang.org/play/?#code/MYewdgzgLgBAZjAvDAPAFQDQD4AUAPALhjRgB8ZoAnASzAHMMYBPIk8sAVwFsAjAU0oBKJFmYAoODgDkAQymMAjMIgyo1CHGp8IMTrwEwA9IeIx1MAEQyLukAHcxY0JFh0kqEnzxQ+YACY6HP58mmB8friEpuzc-EIiMHhidDhKFKrqmtq6sQbGpuZBfiG04bZ2QA

### 💻 Code

```ts
const f = (x: T | string, y: T | number) => y
f('a', 1) satisfies number // T is "a" now

const g = (x: T | number) => x
g(1) satisfies number // T is undefined now
```

### 🙁 Actual behavior

`T` is not `never`.

### 🙂 Expected behavior

`T` should be inferred as `never`, because it can be anything.

### Additional information about the issue

This may be related to #42491.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked TypeScript Playground reproduction and compare its generic inference behavior with the related issue #42491. The work is complete when T is inferred as never in both examples and the satisfies checks produce the expected results.

Written by the indexing model from the issue text.

Assessment

Tech stack
typescript
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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