Quotation does not detect ambiguity

Prerequisites

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[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 the following example, the expr syntax category has two rules, which create an ambiguity: an ident can be directly an expression, or a nullary application. Ideally, this should trigger a warning or an error, but it doesn't, and, although just by looking at the source code, it looks like all the cases of the parse tree are handled, the expression f x reaches the catchall case.

import Lean

open Lean Meta Elab
open Lean.Elab.Command (liftTermElabM)

declare_syntax_cat expr
syntax "#demo " expr : command
syntax:60 ident expr:61* : expr
syntax:90 ident : expr

partial def elab_expr : Lean.TSyntax `expr → CoreM Unit
  | `(expr| $_:ident) => return ()
  | `(expr| $_:ident $args:expr*) => do
    for arg in args do
      elab_expr arg
  | s => throwErrorAt s m!"{repr s}"

elab_rules : command
  | `(command| #demo $expr) => liftTermElabM <| elab_expr expr

#demo f x

Context

See the zulip thread where this issue was first reported.

Versions

Tested on Lean 4.12.0 on a NixOS/nixpkgs unstable computer, and on Lean nightly on live.lean-lang.org.

Impact

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leanprover / lean4