Type check fails when union type has more than 12 literals

[hrjakobsen]

🔎 Search Terms

"12 limit union type", "union type subtyping literals"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about union types

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250123#code/C4TwDgpgBAYglgGwRAJgSQMYHsB2UC8UA3gFBTlRzY4BcUAzsAE5w4DmZFArjgGaLIUAfjq8AhgnrQSAXxIlQkKAFU+A1JlwAVcNEKdyAHygByKrgD6ARhNQDUY2eoWATCfuPzOCwGZ3FB1MvCwAWfwpPZwBWcKMg5wA2WMCnSwB2ZMjLAA5M+MsATjzU7ysABmLgqxsPfNK3WpLrP3lFaFV+JA1qAmJ7LzoO9XRqHUh7Hk7BOmYuCFlW3ShNPEJ4LpHcQKGNlfleHgxgOC2MJggxYAgVgAoBlTVd0d0AGihJ4boAIywsZDEcABKKB0FZ9ALnYBcJh4UgBAJeF72AIfDb2ORyIA

💻 Code

type FilledIcon = {
    icon: string
    unfilled?: false 
}

type UnfilledIconType = 
    | 'icon_1' 
    | 'icon_2'
    | 'icon_3'
    | 'icon_4'
    | 'icon_5'
    | 'icon_6'
    | 'icon_7'
    | 'icon_8'
    | 'icon_9'
    | 'icon_10'
    | 'icon_11'
    | 'icon_12'
    | 'icon_13'

type UnfilledIcon = {
    icon: UnfilledIconType
    unfilled: true
}

type Icon = FilledIcon | UnfilledIcon

function createIcon(icon: UnfilledIconType, unfilled: boolean) : Icon {
    return {
        icon,
        unfilled
    }
}

🙁 Actual behavior

Typescript reports the following error:

Type '{ icon: UnfilledIconType; unfilled: boolean; }' is not assignable to type 'Icon'.
  Type '{ icon: UnfilledIconType; unfilled: boolean; }' is not assignable to type 'UnfilledIcon'.
    Types of property 'unfilled' are incompatible.
      Type 'boolean' is not assignable to type 'true'.(2322)

🙂 Expected behavior

Should type check without problems.

{ icon: 'icon_1' unfilled: true } would be a valid UnfilledIcon, and { icon: 'icon_1' unfilled: false } would be a valid FilledIcon.

Additional information about the issue

This only seems to happen if UnfilledIconType has more than 12 literals. Removing icon_13 causes the type check to succeed.

[jcalz]

This is behaving as designed. Duplicate of #43283 and others. The limit is 25 and you are using your union twice, so 13x2 breaks it.

[hrjakobsen]

Thanks for the clarification and quick response @jcalz!