Type 'string' is not assignable to type 'Exclude<T, number>' when narrowing using generic

[xsjcTony]

Bug Report

🔎 Search Terms

• remove a type from generic union as return type
• remove a type from generic union
• remove a certain type from generic

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about <Can't find anything related>

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.1.0-dev.20230507#code/GYVwdgxgLglg9mABGEBbARgUwE4BU4DKU2MYA5gDy4B8AFAG4CGANiJgFyK4CUnAogA8IrACaYqAGmRos2aogDeAWABQidYmyYoIbEigBPAA6Y4wRE1aZEAXjuIA5Cgw4HiAPyIABgBIFltgBfL0ROAMwAblVA1VUAejjEEABnRjJMVVBIWAREZMxmTGhcTAEoWihSqE5k4lIyRAAfRAAlTDJBI25FVQ1EBMQoOBE4aNiVLOh4JEYwOCgACxwAMTBaRmwyTmdZJry68j22joEunrUNAcWYZMQbxEZoEBZmA0QAdzhsAGt6qXQQFA8nBUJgFnB3ogADKkawAJjut1qMGYzE0jBu9UGS0QOGwX16Gi0Oj0eQKRSgJTKtB2OHwRBI5HWm243CiKhiKnGkxySFpeEIBzIcKodHCnB4EtxZUwYBEt35Hm8flwwVCXHOfWJun0xlM5nCtnsThkriVvn8LCCITCVsiYy5E3AU1y-PpQoAzKKGHaJbwNVVZfLpC5sEraoyGlLlBd1NrSYYTGYLHajTZHPy3J4LeE1barOzOUA

💻 Code

function numberToString<T>(value: T): Exclude<T, number> {
    return typeof value === 'number' ? `${value}` : value;
//  ^^^^^^ Error
}

// usage
function selectText(text: string | RegExp) {
    // todo
}

function anotherFn(arg: number | string | RegExp) {
    // this is actually working, but somehow Line 2 is still raising the error
    return selectText(numberToString(arg));
}

🙁 Actual behavior

Type 'string | T' is not assignable to type 'Exclude<T, number>'.
  Type 'string' is not assignable to type 'Exclude<T, number>'

🙂 Expected behavior

There should be no issue, since T is extending any by default, hence string should absolutely be included in Exclude<T, number>, where it should be any type but without number

[jcalz]

This would probably be a duplicate of #33912 if the return type were changed to T extends number ? `${T}` : T to correspond to what the function is actually doing.

Currently if you call numberToString(10), then T is inferred as 10, making the return type Exclude<10, number> which is never. But the function implementation returns a string, which (like all values) is not assignable to never. So the compiler error is a good one.

[xsjcTony]

@jcalz Thanks for the response. This seems reasonable to me, however I've given it a try but unfortunately it's not working.

https://www.typescriptlang.org/play?ts=5.1.0-dev.20230507#code/GYVwdgxgLglg9mABGEBbARgUwE4BU4DKU2MYA5gDy4B8AFAG4CGANiJgFyK4CUnAogA8IrACaYqAGmRos2aogDeAWABQidYmyYoIbEigBPAA6Y4wRE1aZEAXjuIA5Cgw4HiAPyIABgBIFltgBfL0ROAMwAblVA1VUAejjEEABnRjJMVVBIWAREZMxmTGhcTAEoWihSqE5k4lIyRAAfRAAlTDJBI25FVQ1EBMQoOBE4aNiVLOh4JEYwOCgACxwAMTBaRmwyTmdZJry68j22joEunrUNAcWYZMQbxEZoEBZmA0QAdzhsAGt6qXQQFA8nBUJgFnB3ogADKkawAJjut1qMGYzE0jBu9UGS0QOGwX16Gi0Oj0eQKRSgJTKtB2OHwRBI5HWm243CiKhiKnGkxySFpeEIBzIcKodHCnB4EtxZUwYBEt35Hm8flwwVCXHOfWJun0xlM5nCtnsThkriVvn8LCCITCVsiYy5E3AU1y-PpQoAzKKGHaJbwNVVZfLpC5sEraoyGlLlBd1NrSYYTGYLHajTZHPy3J4LeE1barOzOUA

[MartinJohns]

but unfortunately it's not working.

It's not working because #33912 is not implemented.

where it should be any type but without number

TypeScripts type system can't represent such types. Exclude<> is only used to remove types from a union type. This would require #4196.