we are reporting error for decorators:

case 1206: // ts 3.2 "Decorators are not valid here."

you should enable errorOnTypeScriptSyntacticAndSemanticIssues option

We need to be careful recommending that to Prettier at this stage @armano2 - there’s a major difference in performance to think about right now.

We should measure for the Prettier case specifically, there’s a big user base to consider

I'm working on this now

I've worked on this a bit and we already have precedent for adding decorators on nodes which don't allow them, so I am thinking we should just do that here too to solve them problem for now.

Enabling the diagnostics is expensive, it looks like it's around 30% slower for real world usage of Prettier, because it otherwise doesn't need a lot of the Program architecture.

That might be fine in many cases, might not in others, hard to tell. But given this specific issue, and the fact we already have precedent for this, I think handling it specifically is probably the best way to go.

In the future, it would be really nice if there were a way to cheaply report diagnostics on a single SourceFile, rather than needing a Program cc @typescript-eslint/typescript-team

PR is here: #211

RE: the diagnostics on a single sourcefile, they're technically there, but not exposed in the API. I'll ask around tomorrow to see if that's a safe thing to use/depend on or not