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Function can return wrong values despite type #30603
Description
TypeScript Version: 3.4.0-dev.20190326
Search Terms: Function Generic Extends Keyof Property
Code
interface Type {
a: 123;
b: "some string";
}
function get123<K extends keyof Type>(): Type[K] {
return 123;
}Expected behavior:
The function get123 would yield an error as the return type might not be correct as it depends on K.
Actual behavior:
No error for get123. That function would obviously always return 123.
Playground Link: Example on TypeScript Playground
Activity
jack-williams commentedon Mar 27, 2019
I think this is an unsoundness that exists to support index access types on
this. The assignability rules forType[K]are defined as:The base constraint of
Type[K]is the type you get when indexing intoTypeusing the base constraint ofKwhich is"a" | "b". So the checker sees that123is assignable toType[K]as:123is assignable to123 | "some string"Type[K] ==> Type["a" | "b"] ==> 123 | "some string"The way to prevent the unsoundness would be to write it using two variables, though I'm not sure this actually solves your problem.
EDIT: Just to add the caveat that I think this is what it's doing, and why, but nothing I say is official or guaranteed to be correct.
jack-williams commentedon Mar 27, 2019
See this comment by Anders for (much) more detail.
DanielRosenwasser commentedon Mar 27, 2019
Feels bad - but does that mean we should mark this as a duplicate Anders Hejlsberg (@ahejlsberg)?
jack-williams commentedon Mar 27, 2019
Would it be too much to restrict the unsoundness to cases where the object type is generic, but the key type is concrete? This example would be the converse and use the sound path.
ahejlsberg commentedon Mar 27, 2019
Jack Williams (@jack-williams) I'll give it a try.