Cannot use type narrowing with a generic against a type map

[ArcticZeroo]

🔎 Search Terms

mapped type, map narrowing, generic mapped type, generic narrowing, generic return

🕗 Version & Regression Information

• This changed between versions 3.3.3333 (where it worked) and 3.5.1 (where I see the same errors as on the most recent TS version) based on the playground

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.2#code/KYOwrgtgBAyg9hYAVAngB2AZygbwFBSFQCGANAUQEblFQDGeAvnngC7rBSoZJwBqxADZhgAWWJooAXlwVCAbXiJuWAHTEAugC4o4CJWAAnGkUUJkHTKsraolOHEHBiIEwqUWMVurcytDAJYgAOZMLHRwIH5QaMSGmMACwpwyADxIUMAAHqygACbYHiqYAHwAFABuQiI6foEhpFDsGDpIAJStHLxJIuJo8kga0iWytAEAZlBlzSlSMkWW6m2jtESGwKxghiBQAHKQBoaV1cBtANxyUMyXE1Mz0nOw5sXWy-iraxtbOwBCDk4uY7Jc6Xa5jSbTDgPebPRZ0N6XWjrTbbKBVZIXWhgoisAAWhjgAHddMBiQBRQwEo4AcgAkiB0QE8k0ONSQdcIlFHMBVII4MEyhRYvFEicytSAIzUxoLLxLEzChI9YDiqUy2FyyhtBVxJViyXSp7KOFtPBtIA

💻 Code

enum SomeTypes {
    a,
    b,
    c
}

type TypeToValueMap = {
    [SomeTypes.a]: number,
    [SomeTypes.b]: boolean,
    [SomeTypes.c]: string
}

const parseValue = <T extends SomeTypes>(value: string, type: T): TypeToValueMap[T] => {
    if (type === SomeTypes.a) {
        return Number(value);
    }

    if (type === SomeTypes.b) {
        return Boolean(value);
    }

    if (type === SomeTypes.c) {
        return value;
    }

    throw new Error('Invalid type');
}

console.log(
  parseValue('1', SomeTypes.a),
  parseValue('1', SomeTypes.b),
  parseValue('1', SomeTypes.c)
)

🙁 Actual behavior

All return statements in the parseValue function result in an error that looks something like:

Type 'number' is not assignable to type 'TypeToValueMap[T]'.
  Type 'number' is not assignable to type 'never'.(2322)

This is incorrect because there is no value of T (which must be a member of SomeTypes) that has no mapping into TypeToValueMap, which means that there is no case where narrowing the type should result in never in TypeToValueMap[T].

🙂 Expected behavior

The code as written should have no errors because I am narrowing type to a specific value in SomeTypes, which should provide the TS compiler enough info to map into TypeToValueMap[T].

Additional information about the issue

No response

[MartinJohns]

Duplicate of #43922.

[ArcticZeroo]

Ah, looks like it is, thanks. Followed that to the original issue: #33014 . Looks like it is potentially desired but not currently planned, and that thread has some workarounds.

Will close this one.