Single-line comments can't appear after case name

[pdubroy]

Reported by udob on Discord:

I think I found a problem with line comments in grammars. Checkout this example:

Arithmetic {
  Exp 
    = AddExp        // line comment OK

  AddExp
    = AddExp "+" digit+   -- plus  // line comment after caseName FAILED
    | AddExp "-" digit+   -- minus
    | digit+
}

The second line comment produces a syntax error (expected "}" or "\n"). Changing the line comment to a /* ...*/ makes the error go away.