class GenericClass<T> {
  payload: T;
}

var genericObject = new GenericClass<{greeting: string}>();

function genericFunction<T>(object: GenericClass<T>, callback: (payload: T) => void) {
  callback(object.payload);
}

// Works
genericFunction(genericObject, (payload) => {
  // Here TS understands that payload have a property greeting
  // and that payload.greeting is a string.
});

// Fails to compile with "Type 'T' has no property 'greeting' and no index signature.".
// Type of greeting is any
genericFunction(genericObject, ({greeting}) => {

});