Conditional types produce incorrect typings when used with literal types

[Danchieke]

Bug Report

🔎 Search Terms

Conditional types, union types, literal types, boolean

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type Test<T> = T extends never ? never[] : T[];

let x: Test<boolean> = [true, false];
let y: Test<3|4|5> = [5,3];

🙁 Actual behavior

Compiles with errors

Type 'boolean[]' is not assignable to type 'false[] | true[]'.
  Type 'boolean[]' is not assignable to type 'false[]'.
    Type 'boolean' is not assignable to type 'false'.
Type '(3 | 5)[]' is not assignable to type '3[] | 4[] | 5[]'.
  Type '(3 | 5)[]' is not assignable to type '3[]'.
    Type '3 | 5' is not assignable to type '3'.
      Type '5' is not assignable to type '3'.

x is of type true[] | false[]
y is of type 3[] | 4[] | 5[]

🙂 Expected behavior

x be of type boolean[]
y be of type (3|4|5)[]
Compiles with no errors

[MartinJohns]

This is working as intended. Check the section about distributive conditional types in the handbook.

[Danchieke]

Thank you for that reference, could not find that before.

That being said, this functionality may be intended but is is certainly not intuitive. Is there a way for the compiler to reference the documentation in such cases?

[MartinJohns]

I think identifying the cases where the user mistakingly used distributive types instead of just making another mistake is fairly difficult.

[fatcerberus]

Yeah, there's not really any way for the compiler to guess that you didn't intend to invoke the distributive behavior at the time the type is instantiated.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.

[microsoft-github-policy-service]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow or the TypeScript Discord community.