Writes to indexed access of an index signature are not sufficiently constrained

🔎 Search Terms

"return type" "generic" "type parameter" "indexed access" "index signature" "assignable" "constraint"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about index signatures

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.2#code/C4TwDgpgBAogHgQwLZgDbQLxQN4CgpQIBcUA5AqflANoDWEIJAzsAE4CWAdgOYC6zbLt1wBfXLgAmEAMaoEraNID2nFlAiIU6EvGRoIuZauBR6IAIwCOPKFnKUNe9HQbnetspyUmKUAPR+UF7qrKxKrOJSsvKKKmpmAEwk9lAAPmQARg6a+i4gCe52Xj6k-oEQoeEANIRM6nCQ0sAQEkA

💻 Code

type Example = {
  a: 'a'
  [key: string]: string
}

declare const example: Example
const key1: string = 'a'
example[key1] = 'not a' // no error

declare const key2: 'a' | 'b'
example[key2] = 'not a' // error, as expected
//^^^^^^^^^^^
// Type '"not a"' is not assignable to type '"a"'.

🙁 Actual behavior

The index signature allows any string value to be assigned, ignoring the narrower type of a.

🙂 Expected behavior

Both property assignments in the above code should raise errors, for the same reason.

Additional information about the issue

This is a more generalized version of #60700. @MartinJohns helped me realize that type parameters need not be involved.

Here's another pair of examples (1, 2) which I expected to be reasoned through similarly:

type WiderType = { a?: string, b?: string }
type Example = { a: 'a' } & WiderType
declare const example: Example
declare const key: keyof Example
example[key] = 'not a'
//^^^^^^^^^^
// Type '"not a"' is not assignable to type '"a"'.

type WiderType = { [key: string]: string }
type Example = { a: 'a' } & WiderType
declare const example: Example
declare const key: keyof Example
example[key] = 'not a' // no error

In both cases Example carries with it the information that the property a must have a value assignable to 'a' (as can be seen with example.a = 'not a'). But this is not enforced when assigning to dynamic property using a key whose type matches the index signature.

Here's another example which does produce the error I expect:

type WiderType = { [key: string | symbol]: string }
type Example = { [key: string]: 'a' } & WiderType

declare const example: Example
declare const key: string | symbol
example[key] = 'not a' // error, as expected
//^^^^^^^^^^
// Type '"not a"' is not assignable to type '"a"'.

Comienza con los ejemplos enlazados de TypeScript Playground y compara las asignaciones indexadas mediante claves de tipo string con las asignaciones directas de propiedades. Rastrea cómo el Type Checker determina el tipo escribible para el acceso indexado y añade cobertura que demuestre que se hacen cumplir las propiedades declaradas más específicas; se considera terminado cuando ambos ejemplos reportados rechacen "not a" y los casos válidos existentes sigan aceptándose.