$[$t],* ,$u accepted, but panics when $t is empty list

[sacerdot]

Prerequisites

Please put an X between the brackets as you perform the following steps:

• [x] Check that your issue is not already filed: https://github.com/leanprover/lean4/issues
• [x] Reduce the issue to a minimal, self-contained, reproducible test case. Avoid dependencies to Mathlib or Batteries.
• [x] Test your test case against the latest nightly release, for example on https://live.lean-lang.org/#project=lean-nightly (You can also use the settings there to switch to “Lean nightly”)

Description

In macro_rules, the (`tactic|...) quotation allows to concatenate a possibly empty list with a single element putting a separator in the middle. When the rule is triggered on an empty list, the result is a singleton list that starts with the separator, leading to some panic in tactic code. The following is a minimal example:

syntax "foo " ident,* : tactic

macro_rules
  | `(tactic| foo $[$terms],*) =>
   `(tactic| solve_by_elim only [$[$terms:ident],* ,0])

def foo: True := by
 foo

Steps to Reproduce

Run the code above. The last line triggers the panic

Versions

4.9.0-rc3

[nomeata]

Mildly related: I just leaned that panics are not visible on https://live.lean-lang.org/. I sometimes wonder if panics within commands couldn’t be somehow caught and turned into error, with a location etc.

[nomeata]

Anyways, the panic message here is

PANIC at Lean.Elab.Tactic.SolveByElim.parseArgs Lean.Elab.Tactic.SolveByElim:45:11: Unreachable parse of solve_by_elim arguments.

and it can be reproduced on current master.