Inferring type from Record where keys are template literal types should return correct value type

[minheq]

Bug Report

🔎 Search Terms

• record template literal types [infer]
• record template literal key
• template literal types

Found #29718 but not sure if it is the same error

🕗 Version & Regression Information

TypeScript v4.2.0-dev.20201230

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.2.0-dev.20201230#code/PTAEBcE8AcFNQMIHsA2SBOoC8oDO50BLAOwHNQRQAVAC0N1AHcMBrXAKCjkVQ21AAGAY17oAJAG98RMgF8BnGPGRp0DHACVYI9ABMAPCowAaPARKkAfAG527EcXygRq3AC4er-hPag-oAHIXDAAhFABXWACPALDIgONff0oAZRokcJRdUGh0JABbaHBQWHQ89AB+JL8AkgA3AEMUQl1owK1WxNk7ShCAVQBxADpnUVASADNShnAkQXDiFiRGYgFxx3BYBuykCcFpCwEh+yQN0dV+UlhwAGlYSAA1JsiACmC1UyDROKiASjsJgshOBCKdQFdbvcnhFYPoklQSgAPTbEXQMLQ6AwHMimBYsYjLYiWRJ+G5IlFo0Ase67ajsSwvJAAIwAVh4qKZqZAPDdfuyANo3AC6oB8fnQ13C6GIoGZLP5XKFtm6QA

💻 Code

// type Color = string // This works
type Color = `color${string}`
type Colors = Record<Color, string>;

const colors: Colors = {
    'colorBlue': 'Blue',
    // BUG. should prompt error?
    'invalid': 'Red',
}

// BUG. color infers to `unkown` instead of `string`.
const color = getKeyValue(colors, 'colorBlue')

function getKeyValue<
  T extends Record<string, unknown>,
  K extends keyof T
>(obj: T, key: K): T[K] {
  return obj[key];
}

🙁 Actual behavior

• getKeyValue(colors, 'colorBlue') infers unknown
• 'invalid': 'Red', is still valid key for colors: Colors

🙂 Expected behavior

• getKeyValue(colors, 'colorBlue') should infer string return value
• 'invalid': 'Red', should prompt an error as it does not match template literal type

[andrewbranch]

The definition of Record is

type Record<K extends keyof any, T> = {
    [P in K]: T;
};

which is a mapped type that creates a property for every constituent of K if K is a literal type that can be used as a property name, or a union of those. E.g., Record<'one' | 'two', string> is equivalent to { one: string, two: string }. If K can instead be used as an index signature, it will do that instead, e.g. Record<number, string> is equivalent to { [key: number]: string }. Template literal types cannot be property names (since they can stand for an infinite number of property names), and currently, only string and number can be used as an index signature. So the mapped type can’t do anything and falls back to {}. (You can hover Colors in your playground and see this.)

This will be fixed if #26797 is merged, because template literal types will be allowed as an index signature parameter.

[minheq]

@andrewbranch thanks for clear explanation! i hope the PR will eventually get merged :D

[andrewbranch]

Me too! 😄

[kkmuffme]

@andrewbranch could this issue be reopened?
The PR was superseded by another one, however this "bug" is still here

[andrewbranch]

❓ The expected behavior in the original repro is now the actual behavior.

[kkmuffme]

When I try the playground link above https://www.typescriptlang.org/play?ts=4.2.0-dev.20201230#code/PTAEBcE8AcFNQMIHsA2SBOoC8oDO50BLAOwHNQRQAVAC0N1AHcMBrXAKCjkVQ21AAGAY17oAJAG98RMgF8BnGPGRp0DHACVYI9ABMAPCowAaPARKkAfAG527EcXygRq3AC4er-hPag-oAHIXDAAhFABXWACPALDIgONff0oAZRokcJRdUGh0JABbaHBQWHQ89AB+JL8AkgA3AEMUQl1owK1WxNk7ShCAVQBxADpnUVASADNShnAkQXDiFiRGYgFxx3BYBuykCcFpCwEh+yQN0dV+UlhwAGlYSAA1JsiACmC1UyDROKiASjsJgshOBCKdQFdbvcnhFYPoklQSgAPTbEXQMLQ6AwHMimBYsYjLYiWRJ+G5IlFo0Ase67ajsSwvJAAIwAVh4qKZqZAPDdfuyANo3AC6oB8fnQ13C6GIoGZLP5XKFtm6QA

It doesn't report an error for that?

[andrewbranch]

Yeah, because that’s the version of TypeScript that we were on when the issue was reported 😄

[kkmuffme]

🤣 ah sorry, I thought it would take the latest by default

[andrewbranch]

Haha no worries, I think it depends on whether or not the TS version is explicitly encoded into the URL