Ternary two different types infers to one type when one is a subset of the other

[danielbartsch]

Bug Report

🔎 Search Terms

ternary type union

Maybe related: #33070, #20863

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Generic conditional type T extends never ? 'yes' : 'no' resolves to never when T is never. #31751, but that isn't really it I guess

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = { a: number; b: string }
type B = { a: number }
  
const a: A = { a: 5, b: '' }
const b: B = { a: 10 }

// ab infers as B, should be A | B
const ab = Math.random() > 0.5 ? a : b

🙁 Actual behavior

ab infers to B

🙂 Expected behavior

ab should infer to A | B

[fatcerberus]

This is normal subtype reduction and is by design, as far as I know.

[fatcerberus]

To be fair there is a downside to this kind of subtype reduction: "b" in ab no longer works to recover A.

[andrewbranch]

Yeah, I’ve seen a couple cases like this where subtype reduction is a little unfortunate. Don’t think there’s anything to be done though.

[danielbartsch]

to @fatcerberus, how is this done then?

https://www.typescriptlang.org/play?ts=4.5.0-dev.20211020#code/C4TwDgpgBAglC8UDeUCGAuKA7ArgWwCMIAnAbigMwGdhiBLLAcygF8AoUSKAIQWTUy5CJVmyhQ2AYwD2WGgNh8UGKAFYANBUwBybaJlzgWnkoUBGAAyi2AehtoCUBgDMSVNO+6aqAC2k4AGwATCmg4AB8eKVl5VEpFSN5EAFlUYB8AOmJULCDpPAAKAEooAD4oCwzVKAB+NChMAjY2bQI9BgcoADIuhwyCIA

it doesn't even seem to work when I explicitly say that ab is of type A | B

[MartinJohns]

@danielbartsch In that example the variable ab is still typed B, not A | B. Assigning a value to a variable narrows the type of the variable, and you're assigning a value of type B - so ab is of type B. This behaviour is intentional.

If you want to explicitly type it as A | B you can use a type assertion:

const ab: A | B = (Math.random() > 0.5 ? a : b) as A | B

[fatcerberus]

Well, it would be A | B if it weren't for subtype reduction. That this pattern is broken is exactly what I was pointing out above. Unfortunately, yeah, there's no way to prevent this without a type assertion.

[danielbartsch]

Thank you very much!