Incorrect inference/autocompletion on generic arrays, when values can be inferred from a defined object.

TypeScript Version: 4.1.2

Search Terms: Autocompletion, incorrect, values, inference, generic, array, object, keys

Summary: When an interface/a type has an object with generic keys, and an array of those keys, the array values cannot be infered from the object keys.

Code

interface Recipe<INGREDIENTS extends string> {
  quantities: Record<INGREDIENTS, number>
  allergens?: INGREDIENTS[]
}

function createRecipe<INGREDIENTS extends string>(recipe: Recipe<INGREDIENTS>) {}

createRecipe({
  quantities: {
    eggs: 1,
    flour: 2,
  },
  allergens: ['']
})

Expected behavior: Here, when trying to give a value to allergens, the autocompletion should show "eggs" | "flour".

Actual behavior: The autocompletion doesn't find anything.

Notes: The other way is working: you can fill the array first, then the object keys will autocomplete - but this rarely make sense to write things that way.

Failed workarounds: This bug is still present, even when:

• allergens is optionnal
• We switch from an interface to a type
• We use keyof this['quantities'] instead of INGREDIENTS[]