Typescript infer function parameters in derived class
As the other answer says, this is known issue with TypeScript compiler.
There is a way to provide implementation of MyInterface for which method parameters will be inferred, you just have to use a function that returns an object instead of a class:
interface MyInterface<T> { open(data: T): void}function createMyObject(): MyInterface<string> { return { open(data) { // data type is inferred as string here const n = data.length; } }}