Intersection on indexing a conditional type object #55804

🔎 Search Terms

"indexing a conditional type object", "type mismatch indexing a conditional type object ts"

🕗 Version & Regression Information

• This exists on 5.1.6 and latest (5.2.2)

⏯ Playground Link

https://www.typescriptlang.org/play?#code/KYDwDg9gTgLgBAYwgOwM7wNIFECaBlOAXjgG0ByBAQxjIBo4yATCAczoYDMIR2zore3MgF04lVIhToAsACg5oSLElpMuPAH0AKjgAKWDQFkAgrt0BJAHIBxInADecuImoAuBhDAwAllLpO4ZhZ3Pi9fNH9ZZy4QEM8fP1oA-koQqEpkFmBI5240jKzIgF8xCSRVOQVwaHgYAE8wYDgMYDqANUoAGwBXJuJHKLh48NR3dChvTJJhAPTM4Hd7OABbSYB+d2Ru5YAjYCgAbhXKEA24Ld39uCK5G-lZRRq4esa4cxhgZYl+gJIMOEmcAAFC9gBAOM11ABKEgXPZQYTCM4tdpdXokEENMEQ7D4bR6AwmMxWawwjCI26Ve6dYDwDLeZZdUZvD5fOz2KgwdwkABEIB59B5dQFcD5POEdzkXCgwJp8AA1q0hjjoQ4AgB6dVwbrIcLKuCQVCobw7GlwABuaKaoIkgLIKI6PWyATlcBAdnpjM6qBIirqM3uzk1cCwACVQwB5UPuPZIZbACQZAHID5QVDABAJZBwADuAAtgNmYFA6pMWM8IHB07UCxarQFPUzfa1RMQQJTZEA

💻 Code

export const KEYS = ['cat', 'dog', 'fox', 'orca', 'ox'] as const

export const KEYS_TYPE_MAPPING = {
  cat: 'options',
  dog: 'options',
  fox: 'options',
  orca: 'range',
  ox: 'range',
} as const

export type KeyValue = {
  options: string[]
  range: { min?: number; max?: number }
}

export type Items = {
  [K in (typeof KEYS)[number]]?: KeyValue[(typeof KEYS_TYPE_MAPPING)[K]]
}


let animals: Items = {cat: ["x", "y", "x"]}

for (let key of KEYS) {
  // union of possible value types in 'KeyValue'
  let x = animals[key]

  // ERROR: becomes an intersection when trying to set the value
  animals[key] = x
}

🙁 Actual behavior

After creating a object type where each key has a mapped value type, an intersection of said types is created which causes object[key] to not be the same type as object[key] when setting a value at an index of the object.

It should not be an intersection type since the value types never overlap.

🙂 Expected behavior

I expected the type of 'x' (when setting it as the value of object[key]) to be the same type as 'object[key]'. Since they both reference the same index in the same object. Also I would expect the type of 'x' and 'object[key]' to be a union of the value types since they do not overlap.

Additional information about the issue

No response