TS 4.8 Generics completion is now weaker.

[eretica]

Bug Report
🔎 Search Terms
Generics completion is now weaker.

🕗 Version & Regression Information
This changed between versions 4.7.4 and 4.8.2
⏯ Playground Link

💻 Code

export const func = <T extends string>({ keys }: { keys: T[] }) => {

 const firstKey = keys[0];
 
 return {
 keys,
 firstKey
 } as const;
};

// 4.8.2 firstKey string
// 4.7.4 firstKey "aa" | "bb"
const { keys, firstKey } = func({keys: ["aa", "bb"]})

🙁 Actual behavior

4.8.2 firstKey type of string

🙂 Expected behavior
firstKey type of "aa" | "bb"

[somebody1234]

oddly enough, these both work perfectly fine...

const { keys } = func({keys: ["aa", "bb"]})
const x = func({keys: ["aa", "bb"]})

[Andarist]

On top of that the initial autocompletion for firstKey suggests that it will be of the union type - it just becomes string after the firstKey is being added to the binding pattern and when it becomes part of the contextual node~

I've bisected this change to #49887 by Jake Bailey (@jakebailey)

[jakebailey]

That's... weird. I'll take a look.

[jakebailey]

Ah, so this is the test case that would have caught that we really did need to do something special in getTypeFromBindingElement. No test failed when I removed what I thought was redundant code, but it turns out that this was that case.