Mapped type beyond one level simplification problem with Partial and Record.

[bowenni]

TypeScript Version: 3.4.0-dev.20190202

Search Terms:
Record, Partial

Code

function f<K extends string, V, T extends Partial<Record<K, V[]>>>(
    object: T,
    key: K,
    value: V[],
) {
  object[key] = value;
}

Expected behavior:
No errors.

Actual behavior:
index.ts:6:3 - error TS2322: Type 'V[]' is not assignable to type 'T[K]'.

Playground Link:
https://www.typescriptlang.org/play/#src=function%20f%3CK%20extends%20string%2C%20V%2C%20T%20extends%20Partial%3CRecord%3CK%2C%20V%5B%5D%3E%3E%3E(%0D%0A%20%20%20%20object%3A%20T%2C%0D%0A%20%20%20%20key%3A%20K%2C%0D%0A%20%20%20%20value%3A%20V%5B%5D%2C%0D%0A)%20%7B%0D%0A%20%20object%5Bkey%5D%20%3D%20value%3B%0D%0A%7D
Enabling all strictness flags still gives me the same error.

Related Issues:
I think this is a duplicate of #28839, which is marked as fixed. I asked for reopen in #28839 (comment) but got no rely. I'm assuming that closed issues are not on your radar so opening a new issue.

[ahejlsberg]

This is working as intended. In your example, a type argument for T can be a subtype of Partial<Record<K, V[]>>, so properties of T might actually be subtypes of V[] (e.g. a specialized array with additional properties). The type safe way to write your example is:

function f<K extends string, V>(
    object: Partial<Record<K, V[]>>,
    key: K,
    value: V[],
) {
  object[key] = value;
}

This compiles without errors.