Unexpected broadening of generic type parameter when used in the types of 2 different function params

[estradanic]

🔎 Search Terms

type broadening, type narrowing, generics, generic type parameter, index type, template literal, typescript, unexpected, why doesn't typescript know that these are the same, same generic instance not enforced between function params

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about the same generic instance type being used in different function parameters

⏯ Playground Link

https://www.typescriptlang.org/play#code/C4TwDgpgBAyg9gWwsAFgSwHYHMoF4oDk8SqmWBA3AFCiSyLLrYCiANgM7T5EOkscRKVGuGgAVCAjCsAhsAhjReKAANijMgDoAJAG91fLAF8VUAD6qDTLG0479va7YgnhAMwCuGAMbA0cDCgAEzgrMgB1NFQAWRAxawAeMSgIAA95DCD2ehJrcxyNfk4APgAKNzQIViCALigxABooODA-ALqxAEooXSoofqhvAPZgKAQ4yWk5CA7J2XlFOnwVPQqqoKN7FraME2ojKiA

💻 Code

type Something = 'Something';
type SomethingElse = 'SomethingElse';

type TemplateType = `Something.${Something}` | `SomethingElse.${SomethingElse}`

function doSomethingWithMyThing<T extends Something | SomethingElse>(field: T, option: T) {
    const myTemplate: TemplateType = `${field}.${option}`;
}

🙁 Actual behavior

The actual type of ${field}.${option} is some sort of cartesian product of the types of field and option, causing the TemplateType type of myTemplate to be incorrect.

🙂 Expected behavior

The type of ${field}.${option} should be either "Something.Something" or "SomethingElse.SomethingElse" based on the same T being enforced between the two params.

Additional information about the issue

No response

[fatcerberus]

See #57372 (comment).
T can legally be instantiated to Something | SomethingElse in which case the parameters don't have to correlate. You're looking for #27808.

[estradanic]

Interesting. That is frustrating, but it makes sense. Thanks for the link!