Union type not correctly narrowing down

[danielbartsch]

Bug Report

🔎 Search Terms

union narrowing wrong

🕗 Version & Regression Information

in 3.7.5. still there in 4.3.2

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "bugs that aren't bugs"
• I was unable to test this on prior versions because prior versions don't support ??

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = { a: number, b: string }[]
type B = { a: number }[]

declare var c: A | B | undefined;

const d = c ?? []

// d is now just B, but should be A | B

🙁 Actual behavior

Type inference excludes a part in the intersection type.

🙂 Expected behavior

Type inference should just exclude undefined, because the ?? operator was used on the variable.

[fatcerberus]

That's a union type, not an intersection.

[danielbartsch]

whoops. You're right. With that correction, I was able to find related issues:
#44401
#43825
#43667

which are almost the same, but not quite.

[MartinJohns]

See: #32237 (comment)