const patterns can't be used by themselves, need // prefix

[jaqx0r]

When using a const by itself as a match condition, then we get warnings about
the capture group names not existing.

    const A /(?<a>.*)/
    A {
      x[$a]++
    }

reports that $a not defined in scope.

A workaround is to prefix A with a // to force it into a locally evaluated pattern, which is weird:

// + A {
  x[$a]++
}