Object spread unsound w.r.t. width subtyping of object types

[samwgoldman]

TypeScript Version: Whatever is running on TypeScript Playground

Code

var o1: { p: string, q: number } = { p: "", q: 0 };
var o2: { p: string } = o1;
var o3: { p: string, q: string } = { q: "", ...o2 };

Expected behavior:

Some type error somewhere. o3 has type {p: string, q: number} at runtime, but we think q has type string.

Actual behavior:

No error. Potential runtime type error.

[RyanCavanaugh]

"somewhere" is doing a lot of work there 😉 . Where specifically should the error be (and by what process)?

[samwgoldman]

Hah, fair! I thought the example was small enough to make it clear :)

One option would be to forbid width subtyping between object types, so the error would be on line 2. This is pretty limiting, so not a great option.

Another option would be to error on line 3. Because the q property of o2 could be anything as far as TS knows, you could infer the type of the spread to be { p: string, q: {} }, which would be incompatible with { p: string, q: string }.

[RyanCavanaugh]

Without closed types, that's effectively the same as "don't allow spreading, ever", which doesn't really seem helpful.

[samwgoldman]

Just confirming this is by design. (I agree this is a reasonable tradeoff to make, FWIW)

[RyanCavanaugh]

Yep

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.