Typescript: False positive error report for interface type missing index signature, thus not assignable to object type with `unknown` values.

[woutervh-]

TypeScript Version: 3.6.0-dev.20190704 (3.4.3 doesn't have this bug)

Search Terms:
index signature, unknown, assignable, interface

Code

interface A {
  properties?: { [Key: string]: unknown };
}

type Bar = {
  key?: "value";
};

interface Baz {
  key?: "value";
}

interface B extends A { // ok
  properties?: Bar;
}

interface C extends A { // error: Baz is not assignable to { [Key: string]: unknown } -> Index signature is missing
  properties?: Baz;
}

Expected behavior:
No error expected, interface C extends A correctly.

Actual behavior:
Error because interface C does not extend interface A

Playground Link:
http://www.typescriptlang.org/play/#code/JYOwLgpgTgZghgYwgAgILIN4ChnIA5QD2e0YwEAzgPwBcmyA2gNIQCedFYUoA5gLp0AriADWIQgHcQyAL4BuLDKxYwrEsgBCcKMgC8mHMhFtayAEQA3OABtBEMwvnLQkWIhRaAXgdzHWpyxs7B0VncGh4JE1kCAAPSBAAEwo0egB6NORCEUMCYlJyajotKEcw10iUAGEY+IgklPQMZAyYqCIoYrhvYBTxMGQ4CgpgHhA4ACNrFDBCemY2Di5eAWRhMUlpGWQAWgA+ZABJJLjkEbG4MEEoFF7kAFtekZAeXKISKDJKUy8yoA

Related Issues:

30977 (this one is specifically for enums)

[jack-williams]

I believe this is working as intended: object literal types have implicit index signatures, while interfaces do not. See the implementation here: #7029

[woutervh-]

Thanks @jack-williams , I didn't know about this specific assignability rule. Do you know if there is a comprehensive list of type assignability rules out there? I tried to search for one but couldn't find.

[jack-williams]

Not really - If you want to know the absolute truth you need to go here:

• https://github.com/microsoft/TypeScript/blob/master/src/compiler/checker.ts

The spec is pretty good, although incomplete:

• https://github.com/microsoft/TypeScript/blob/master/doc/spec.md#3114-assignment-compatibility

And something to keep an eye on:

• https://github.com/microsoft/TypeScript-New-Handbook

[fatcerberus]

I hate that checker.ts is so absolutely ginormous that GitHub refuses to display it - it makes it impossible to link to specific lines.

[jack-williams]

The reason intellisense for TypeScript is so performant is because the TypeScript team want to have auto-completion while editing checker.ts[1] - so if anything, we should be encouraging them to make it larger.

_{[1] I made that up}

[RyanCavanaugh]

I need to add this to the interfaces vs type aliases section.

... if anything, we should be encouraging them to make it larger.

I feel seen. Also we should make the binder incremental.