Indeed, there have been numerous reports regarding this issue ever since TypeScript 3.5 was released. The enhanced soundness of indexed accesses, as highlighted in the breaking change documented here, has resulted in catching real bugs but also triggering warnings on relatively safe code practices.
The root problem lies in the fact that generic type parameters extending unions do not undergo narrowing via control flow analysis (refer to microsoft/TypeScript#24085). This limitation prevents the compiler from inferring that if p is narrowed down to "a", then the type parameter K should be similarly narrowed to "a". The current language lacks certain functionalities that would facilitate this kind of inference securely.
As a temporary solution, workarounds and refactoring techniques can be employed. One potential workaround, suggested by @TitianCernicova-Dragomir in this answer, involves using a single-call-signature overload function to revert back to the behavior seen in TS3.4 or earlier. While this approach may introduce some unsafety, it counteracts the changes implemented in TS3.5. Other possible workarounds, such as type assertions or alternative unsound methods, may also prove effective.
In light of the limitations with generics and control flow narrowing, a refactoring strategy devoid of such expectations could prove beneficial depending on the scenario. Consider returning something recognizable to the compiler as a legitimate Foo[K] by accessing an object of type Foo with a key of type K.
function getProp<K extends keyof Foo>(o: Foo, p: K): Foo[K] {
return { a: 1, b: o.b }[p];
}
Access Code Here