Cases on mvar leading to crash/stackoverflow

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• 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

example : ∃ c, c = true := by
refine ⟨?c, ?_⟩
cases ?c with
| true => sorry
| false => sorry

The following program seems to stackoverflow/crash both locally trying multiple versions of lean/lean lsp and the lean live on nightly.

(For example it might stall for a very long time, report "Lean server printed an error: Stack overflow detected. Aborting.", or "Server process for file:///.../test_rbtree_typecheck_timeout.lean crashed, likely due to a stack overflow or a bug. Code: -32902"

Context

[Broader context that the issue occurred in. If there was any prior discussion on the Lean Zulip, link it here as well.]

Steps to Reproduce

1. Try to run the above minimized lean program.

Expected behavior: [Clear and concise description of what you expect to happen]

Make forward progress.

Actual behavior: [Clear and concise description of what actually happens]

Lean seems to crash/become non-responsive.

Versions

[Output of #version or #eval Lean.versionString]
[OS version, if not using live.lean-lang.org.]

https://live.lean-lang.org/#project=lean-nightly&codez=ATCmA8EMFsAcBtTAFzEMBEwDGAaLwC8wALgE4CuSyhARgJ4BQIIJoAZgJYB2SgF%2BQD8OYHwD6gS%2FJGTTJADOoaUMzAA7uyIALCSAA%2BxcknwA%2BYNID2JEgybAdrSPFkEjp87SA

Additional Information

I think the closest issues I found were https://github.com/leanprover/lean4/issues/2504, https://github.com/leanprover/lean4/issues/4219, https://github.com/leanprover/lean4/issues/2506 though I would not say any of these directly apply.

Impact

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Start by running the minimized Lean program from the issue against the Lean nightly release or the linked live.lean-lang.org example, confirming the stack overflow or crash when cases is applied to the metavariable. Trace the elaboration path involved and add a regression test for this program; done means it makes forward progress without crashing or becoming non-responsive.