Support for new RHS parameter forms

[bterlson]

There is a proposed ECMAScript change to RHS parameter forms. Specifically, if when an RHS non-terminal production is parameterized, the RHS must be explicit about whether it is passing the parameter (currently an unadorned parameter name), forwarding the value of the parameter passed into the production (currently prefixed with ?), or not passing the parameter. Specifically:

Form	Current	New
Set parameter	Foo	+Foo
Forward parameter	?Foo	?Foo
Don't set parameter	Parameter is omitted	~Foo

Example

Foo[Param] :
  [+Param] `Present`
  [~Param] `Not present`

Bar[Param] :
  Foo[?Param]

Baz :
  Foo[+Param]

Qux :
  Foo[~Param]

[rbuckton]

I've added partial support for parsing this for now. I'd like to add some additional checking to report diagnostics if you don't pass a parameter explicitly.

[bterlson]

Possible to publish (even a beta) without the checking functionality?

[bakkot]

@rbuckton I suspect this can be closed?

[rbuckton]

👍