Question: what the AST should be

[fisker]

There is a test case in Prettier

https://github.com/prettier/prettier/blob/d932cf70dfa96357acae2568f7989bfa2f1e6360/tests/format/js/decorators/member-expression.js#L18-L21

[
  class {
    @decorators[0]
    method() {}
  }
]

Currently flow-parser and meriyah parse it as method with decorators[0] as decorator. While typescript and babel parse it as computed 0 property with decorators as decorator.

Which one is expected?

[nicolo-ribaudo]

You can find the syntax spec at https://arai-a.github.io/ecma262-compare/?pr=2417&id=sec-decorators

The relevant part is:

Decorator :
 `@` DecoratorMemberExpression
 `@` DecoratorParenthesizedExpression
 `@` DecoratorCallExpression

Where DecoratorParenthesizedExpression is in parentheses, DecoratorCallExpression is a DecoratorMemberExpression followed by parentheses for the call, and DecoratorMemberExpression is

DecoratorMemberExpression :
  IdentifierReference
  DecoratorMemberExpression . IdentifierName
  DecoratorMemberExpression . PrivateIdentifier

Which means that decorators cannot contain [0] (unless they are a DecoratorParenthesizedExpression). So in that example the decorator is @decorators and not @decorators[0].

[fisker]

Thank you! So flow-parser and meriyah are wrong.

Do you know if test262 covers this case?

[nicolo-ribaudo]

Do you know if test262 covers this case?

Unfortunately test262 has terrible coverage for the decorators proposal, so probably no. It's best to look at this PR: tc39/test262#4103