Generic decorators - could they receive some default type arguments?

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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(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 => 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(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).

L’issue fornisce esempi di decorator e argomenti di tipo generici, ma non indica file del repository, test o punti di ingresso. Inizia individuando le aree del compilatore che verificano i tipi dei decorator e inferiscono gli argomenti generici, quindi definisci il comportamento supportato e i test necessari per verificare l’inferenza type-safe del target e del valore.

Scritto dal modello di indicizzazione a partire dal testo della issue.