We could do OCL based Post and Preconditionals, but let's see how hard it would be to do native preconditionals.

OCL conditionals operate with contextual access to a method's arguments, result, and for postconditional, values of the arguments prior to execution, pre@.

To replicate this using TypeScript decorators, we would need the same access to the arguments as OCL does. This can be (poorly) replicated using a custom generic type that extracts the arguments of a function as a tuple list.

type ArgumentTypes<F extends Function> = F extends (...args: infer A) => any ? A : never;

We can then define a decorator function like so...

export function Preconditional<TArgs>(expr: (args: TArgs) => boolean) {...}

We can then define a Preconditional on a method

export class Student {
    @Preconditional<ArgumentTypes<typeof Student.addCourse>>((args) => args[0] > 0)   
    public static addCourse(courseId: number) {...}
}

The above example works for Preconditions on the arguments, but what about the larger environment? Postconditionals are often used to validate not only the return value, but side effects that the function generated, such as a change to an instance field.

Some notes:

1. The decorator invocation is ugly, very ugly.
2. The ArgumentTypes gives us a tuple with argument positions and types, not the argument names. More ugly.

TypeScript changes to support generic decorators.

The above sections prove that preconditions and postconditionals can be implemented without changes. However, the required boilerplate is highly verbose and prone to error. A solution would be for the typescript language service to pass two generic parameters whenever it sees a generic function. This would bake in this functionality. But perhaps we could be more cautious, and just define this as the target type of the decorator. This would result in much simpler definitions, such as

@Preconditional<ArgumentTypes<this>>((args) => args[0].courseNumber > 5)

So in the above example, this would be logically equivalent to typeof Student.addCourse