Code completion stopped working for a few scenarios in typescript 5.1

[pedrodurek]

Bug Report

Scenario 1: Code completion stopped working by adding an undefined as the default value for the generic type. TypeScript narrows down the range of expected argument types, and as a result, the code completion doesn't work correctly. It works fine in Typescript 5.0 or below.

Scenario 2: Code completion stopped when using an overloaded function type that combines union types with named tuples containing a generic type parameter. it works fine in Typescript 5.0 or below.

🔎 Search Terms

Code completion issues in typescript 5.1

🕗 Version & Regression Information

The Issue started occurring on typescript 5.1 and beyond. It works fine in versions 5.0 or below.

⏯ Playground Link

Scenario 1: https://tsplay.dev/w1xZGW
Scenario 2: https://tsplay.dev/WKvGZm

💻 Code

Scenario 1:

declare function func<
 const T extends 'a' | 'b' | undefined = undefined,
>(arg?: T): string;

// Code completion does not work
func('')

Scenario 2:

interface Func {
 <Key extends 'a' | 'b'>(
 ...args:
 | [key: Key, options?: any]
 | [key: Key, defaultValue: string, options?: any]
 ): string;
}

declare const func: Func;

// Code completion does not work
func('');

🙁 Actual behavior

Code completion doesn't work

🙂 Expected behavior

Code completion should work as in the images below:

[Andarist]

Both are already fixed by #54588 and that PR is being cherry-picked into 5.1 release line here.