pattern(go): ellipsis does not match cases with one-or-more case clauses

[craigfe]

Describe the bug

The ellipsis operator cannot be used to match sequences of case clauses in a Go switch statement. For example, I'd expect the following:

switch $_ {
  case 0, ...:
    ...
}

To match the following examples:

func main() {
    // ruleid: ellipsis-in-case
    switch n {
        case 0: return
    }

    // ruleid: ellipsis-in-case
    switch n {
        case 0, 1: return
    }

    // ruleid: ellipsis-in-case
    switch n {
        case 0, 1, 2: return
    }
}

To Reproduce
https://semgrep.dev/playground/s/BY0ob

What is the priority of the bug to you?

• P0: blocking your adoption of Semgrep or workflow
• P1: important to fix or quite annoying
• P2: regular bug that should get fixed

Use case
This would allow us to strengthen our Semgrep rules that attempt to catch branching on certain constants (using either switch or if). Our existing workaround is to use pattern-either to enumerate patterns for a fixed number of case clauses (e.g. 0-8).