BREAKING: throw when invalid modifiers are added to parameter properties

[j-f1]

BREAKING CHANGE: static and exports modifiers are no longer permitted on parameter properties

These result in a compile-time error, as shown in the playground.

[j-f1]

This would be covered by bradzacher/eslint-plugin-typescript#245 & eslint/typescript-eslint-parser#547 (comment). Feel free to close if those solutions are preferred.

[JamesHenry]

@j-f1 I'm curious about "no longer permitted" and then providing a playground showing type check errors.

This for me just confirms that it has always been the case in TypeScript that things like this are parseable and so therefore produce an AST. TypeScript then uses that to inform the separate checking step.

As a result other parsers in the ecosystem are a lot less forgiving than tsc and provide immediate feedback about syntactic and semantic issues.

That is not to say I am against moving away from the "loose" behaviour towards consistency with other parsers (particularly babel), I was just curious about you implying that something had changed recently about this behaviour...

As you mentioned a couple of things have been proposed, and I think maybe starting with just offering an option to throw on semantic errors we can retrieve from the TypeScript program would be a great start.

If that is not enough in some cases we can add in special handling as you have done here (although again, probably behind the new option).

I will put together a PR for that now, and then we can see if we want to press ahead with this one on top.

Sound good?

[j-f1]

Sounds good. Throwing the syntactic and semantic errors will probably make this PR irrelevant.