A decorator can receive arguments, by being defined as a function that accepts parameters and then returns a normal decorator function:

function computed(evaluator: (obj: any) => any) {
    return (obj: any, key: string) => {
        Object.defineProperty(obj, key, { 
            get() { return evaluator(obj); }
        });
    };
}

I could use the above example like this:

class C {
    firstName = "Homer";
    lastName = "Simpson";

    @computed(c => c.firstName + " " + c.lastName) 
    fullName: string;
}

Not a realistic example in that it's just another way of defining a property getter, but it's enough to demonstrates the general problem: c is of type any so the access to firstName and lastName is not type-checked, and nor is the result of the expression.

We can attempt to address this manually, because decorators can be generic:

function computed<O, R>(evaluator: (obj: O) => R) {
    return (obj: O, key: string) => {
        Object.defineProperty(obj, key, { 
            get() { return evaluator(obj); }
        });
    };
}

class C {
    firstName = "Homer";
    lastName = "Simpson";

    @computed<C, string>(c => c.firstName + " " + c.lastName)
    fullName: string;
}

But this still doesn't close the loop. The compiler is happy with the following abuse, in which c is supposedly a string but is actually a C, and fullName is declared a string but will actually become a number (albeit NaN I guess):

@computed<string, number>(c => parseInt(c, 10))
fullName: string;

In summary: decorators are passed the values of the meta-information in standard parameters (target, key, value), but they are not passed the compile-time types of those values in a way that can be used to create fully type-safe decorators.

So it would be helpful if in decorator-defining functions with generic parameters, those parameters could somehow map automatically to the types of target and value (as given meaning in the __decorate helper).

For big yucks, a straw man in which the compiler recognises a couple of built-in decorators that can appear on the type parameters:

function computed<@decoratorTarget O, @decoratorValue R>(evaluator: (obj: O) => R) {
    ...
}

Being decorated in this way, it would not be possible to manually specify type arguments for those type parameters when using the decorator. They would effectively be invisible to the user. If the decorator has its own custom type parameters, they would appear from the outside to be the only parameters (and for clarity should be at the front of the list).