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"'.

Start with the linked TypeScript Playground examples and compare indexed assignments through string keys with direct property assignments. Trace how the type checker determines the writable type for indexed access, then add coverage showing that narrower declared properties are enforced; done means both reported examples reject "not a" while the existing valid cases remain accepted.