Exclude<string, "foo"> is not working

[frodo821]

TypeScript Version: 3.8.3 or earlier

Search Terms: Exclude, Conditional Types

Code

type Foo = Exclude<string, "bar">;

// this should cause an error.
const baz: Foo = "bar";

Expected behavior:
"bar" is not assignable to baz.

Actual behavior:
"bar" is assignable to baz without any errors.

Playground Link: https://www.typescriptlang.org/play/#code/FAFwngDgpgBAYgewTAvDAogDwMYBsCuAJlADwDOIATgJYB2A5gDQwBEARgIaUsB8A3MGDYEtCjE4AvAFzwkqVp24ChIsglxQAdLgT0AFJICUAoA

[stormpat]

Not the most elegant, but here is a work-around.

https://www.typescriptlang.org/play/?ssl=4&ssc=1&pln=5&pc=1#code/C4TwDgpgBAQghgOwRAJlAvFARAIzgJywG4AoUSKAMQHtqMoBRADwGMAbAVxQgB4BnYPgCWCAOYAaWImQoAfKRItqCAVDwAvKAC4qtergLEoAemNU4Qtn110h1uFAHCxJEgDMOCFsCHKo1AGseABUoCCZgCAQUaycRUVkACgA3HVCAMihE0PDI6Ot4JFQoAH5sGABBADkqhgARLB0AbwBfAEo2qCaSKCh8CGAOfAQoZNIW1yUVYDU4dQB9Nws2ekDEg0JO03NLPkVlVQ15wNWA9YRqebxNkzMAYQALCBYAviA

Edit.

Another solution:

https://www.typescriptlang.org/play/#code/MYewdgzgLgBA8gBygS3BGBeGAeUkoB8MA2gOQBmIIpANDKQEYCGATqQLoDcAUFAJ4IApvADWmGPyEhy8JKkjEwAVwC2DQSy7deA4QDkQUAEKtxAUQAewADZKAJoOxwRdAETMWrgj255oMDxgALhgDY1Msd1ZXThgAejiYADEmZGt0MJMWGBUlfzBDAOEoz180WA8AfRAxEMyImFdKEBj4xOc6euzkdALYEtduIA

[IllusionMH]

This is limitation of Exclude. Basically it works when you "filter" unions, but string is not a union.

What you are lookin is Negated types: #29317 and comment about your use case #29317 (comment)

[frodo821]

Thank you for comments!
I guess it means that, in the type system of typescript, string type is not a 'representative' or 'set' of any valid strings.
I thought string means the set of any of it including 'foo', 'bar' or any others of string.

[DanielRosenwasser]

Yeah, unfortunately we're not wired up to "cut holes" out of infinitely large domains of types, and it appears that adding that assumption to the type system would make it incredibly painful to use.

Regarding

https://www.typescriptlang.org/play/?ssl=4&ssc=1&pln=5&pc=1#code/C4TwDgpgBAQghgOwRAJlAvFARAIzgJywG4AoUSKAMQHtqMoBRADwGMAbAVxQgB4BnYPgCWCAOYAaWImQoAfKRItqCAVDwAvKAC4qtergLEoAemNU4Qtn110h1uFAHCxJEgDMOCFsCHKo1AGseABUoCCZgCAQUaycRUVkACgA3HVCAMihE0PDI6Ot4JFQoAH5sGABBADkqhgARLB0AbwBfAEo2qCaSKCh8CGAOfAQoZNIW1yUVYDU4dQB9Nws2ekDEg0JO03NLPkVlVQ15wNWA9YRqebxNkzMAYQALCBYAviA

you might be able to get away with using never in place of BANNED