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Function can return wrong values despite type #30603
Description
Activity
I think this is an unsoundness that exists to support index access types on
this. The assignability rules forType[K]are defined as:// A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C, // where C is the base constraint of T[K]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"- and
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.
function get123<K extends keyof Type, T extends Type = Type>(): T[K] { return 123; }
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.
Reacted by Ryan Cavanaugh and Bjørn ArnholtzSee this comment by Anders for (much) more detail.
Reacted by Ryan CavanaughFeels bad - but does that mean we should mark this as a duplicate Anders Hejlsberg (@ahejlsberg)?
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.
Jack Williams (@jack-williams) I'll give it a try.
Reacted by Jack Williams and Bnaya Peretz
TypeScript Version: 3.4.0-dev.20190326
Search Terms: Function Generic Extends Keyof Property
Code
Expected behavior:
The function
get123would yield an error as the return type might not be correct as it depends onK.Actual behavior:
No error for
get123. That function would obviously always return 123.Playground Link: Example on TypeScript Playground