Type string is assignable to interface with index signature from number to string

[vostrnad]

TypeScript Version: 4.1.0-beta

Search Terms: string assignable index signature number

Code

interface NumberIndexable {
  [index: number]: string;
}

function setValueAtIndex(object: NumberIndexable, index: number, value: string) {
  object[index] = value;
}

setValueAtIndex({}, 0, 'test'); // works fine
setValueAtIndex('oops', 0, 'test'); // runtime error

Expected behavior:

Second call to setValueAtIndex should raise a compiler error.

Actual behavior:

No compiler error is raised, which results in a runtime error because string indeces are read-only.

Playground Link: Playground

[MartinJohns]

string being assignable to your type is correct and working as intended. A string can be accessed with an index signature.

The problem stems from TypeScript not distinguishing between readonly and mutable types, or no way to specify immutability in the type-system. The immutable type string should not be assignable to a mutable interface. Issues to track this would be #18770 and #10725.