Exclude<string, SomeType> not working as expected

[NiloCK]

Possibly this lies between being a Bug and being a blind spot in the documentation (Handbook, in this case), but I can't see any indication in the specification of Exclude that it only works to exclude individual types from a union type. I might guess that this is the case from the provided examples, but probably that's not the intent of the documentation.

Here is a minimal example of what I'm referring to:

Bug Report

🔎 Search Terms

• exclude utility type
• exclude from string
• exclude from type

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about bugs that are not bugs_
• I was unable to test this on prior versions because time

⏯ Playground Link

Playground link with relevant code

💻 Code

type T0 = Exclude< string, "a">;
//    ^?  (== string)
type T1 = Exclude< "a" | "b", "a">;
//    ^?

let a0 : T0 = "a" // should be excluded?
let a1 : T1 = "a" // works ( ie, the assignment does not work. The _exclude_ works )

🙁 Actual behavior

Exclude did not exclude the supplied ExcludedUnion ("a") from string.

🙂 Expected behavior

Exclude excludes "a" from the defined type. From the handbook:

Exclude<Type, ExcludedUnion>
Constructs a type by excluding from Type all union members that are assignable to ExcludedUnion

[fatcerberus]

Exclude can’t work on string because there’s no way to represent string & ~"foo" in the type system. It only works to exclude types from a union type.

[NiloCK]

I am finding this meaning in the wording of the handbook now - it's reference to union members refers both to the (implicit) union members of Type and the union members ExcludedUnion - not just the latter.

there’s no way to represent string & ~"foo" in the type system

... because I'm here - why not? Type system is OK with unions and intersections. What does ~ break?

[fatcerberus]

... why not? Type system is OK with unions and intersections. What does ~ break?

Q. Why isn't this feature supported?
A. Because nobody implemented it yet.

It doesn't break anything - it's just not something that's supported right now. See #29317.

[JustFly1984]

I would love to have a type string & not ""
Also number & not 0 would be great

[RyanCavanaugh]

Exclude, being a conditional type, is distributive over unions, which is why it works, but TS doesn't have any notion of an "infinite union" to represent all strings.

[NiloCK]

Closing via microsoft/TypeScript-Website#2192