How to modify a type in a decorator?

[vpzomtrrfrt]

The documentation on creating decorators describes how to set metadata, but not how to modify the type itself.

How is that meant to be done? Is it safe/meaningful to modify the target value or do I need to return a new type somehow?

[timotheeguerin]

You can just modify the decorator target. We do that for a few decorators

typespec/packages/compiler/src/lib/decorators.ts

Line 932 in 969d380

filterModelPropertiesInPlace(target, (prop) => pickedNames.has(prop.name));

Is it safe?

It depends on what you are doing. If you are just modifying exactly that target and its direct properties it is most likely fine. If however you start mutating sub types or other things it will very likely have issues.
One of the main problem is that you never know what as been fully resolved by the time the decorator run. When types reference each other it causes certain things to be resolved before others which can cause decorators that expect too much to be ready to fail.

[vpzomtrrfrt]

So what if I do want to modify the types it references?

Ultimately, what I'm trying to accomplish is intersecting a union with a model, which doesn't seem to be directly supported

[timotheeguerin]

that's where you might get into issue depending on the order things are referenced.

@myDec
model A {
  b: B;
  prop: string;
}

@myDec
model B {
  a: A;
  prop: string;
}

like in this case there will be at least one of those 2 run of myDec where you won't already have fully checked the other type, in this case the property prop for example wouldn't be set yet.