Cover grammar application should be more discoverable

[Richard Gibson]

There are several parts of the specification that make use of cover grammars, but the details can be buried. The most obvious example is at Left-Hand-Side Expressions, where there is a CallExpression_{[Yield, Await]} : CoverCallExpressionAndAsyncArrowHead_{[?Yield, ?Await]} production but no indication of the supplemental CallMemberExpression_{[Yield, Await]} : MemberExpression_{[?Yield, ?Await]} Arguments_{[?Yield, ?Await]} refining its CoverCallExpressionAndAsyncArrowHead that appears after a lot of intervening productions.

I can think of a few ways to address this, and they're non-exclusive:

• Promote "Supplemental Syntax" into actual subsections so they appear in the table of contents.
• Annotate productions to indicate that they are subject to refinement (may require changing grammarkdown).
• Give each cover grammar goal symbol a dedicated subsection that references its refinements, such that e.g. clicking on |CoverCallExpressionAndAsyncArrowHead| in the |CallExpression| productions goes to a section that explains its use in both |CallExpression| (with a link to |CallMemberExpression|) and |AsyncArrowFunction| (with a link to |AsyncArrowHead|).

[jmdyck]

Annotate productions to indicate that they are subject to refinement (may require changing grammarkdown).

The production could even say what the refinement is. E.g.:

PrimaryExpression : CoverParenthesizedExpressionAndArrowParameterList \
                              [must cover a ParenthesizedExpression]
 
ArrowParameters : CoverParenthesizedExpressionAndArrowParameterList \
                              [must cover an ArrowFormalParameters]

which would give you the supplementary nonterminal to click on.

For the LeftHandSideExpression cases, the must-covers are conditional, so you could say something like:

AssignmentExpression : LeftHandSideExpression \
     [may be required to cover an AssignmentPattern] `=` AssignmentExpression

(or briefer words/symbols to that effect), with the understanding that the condition under which it must cover is spelled out in prose nearby.

Conceivably, we could devise meta-syntax to express the condition in the annotation, but it's probably not worth the bother.

[Richard Gibson]

Yes, that's the sort of thing I had in mind and that specific approach is already possible with prose assertions:

CallExpression[Yield, Await] :
  CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await] [> (must cover a |CallMemberExpression|)] #callcover
  …

CallExpression_{[Yield, Await]} :
     CoverCallExpressionAndAsyncArrowHead_{[?Yield, ?Await]} (must cover a CallMemberExpression)
    …