Prerequisites

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

• 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

Consider this code:

example (l m : List Nat) (i : Nat) (hi' : i < (l ++ m).length)
    (hh : ∀ hi, l[i]'hi = 5) : (l ++ m)[i] = 5 := by
  rw [List.getElem_append_left]
  · sorry
  · sorry

After the rw, there are two goals: l[i] = 5 and i < l.length. Now, if I replace the first sorry with rw [hh], then two unexpected things happen: first, the rewrite leaves behind a goal of i < l.length (why was this not filled by unification?), and second, the second goal below disappears:

example (l m : List Nat) (i : Nat) (hi' : i < (l ++ m).length)
    (hh : ∀ hi, l[i]'hi = 5) : (l ++ m)[i] = 5 := by
  rw [List.getElem_append_left]
  · rw [hh] -- Error: unsolved goal `i < l.length`
  · sorry -- Error: No goals to be solved

Here is a video of the behavior: https://www.youtube.com/watch?v=JpYr1tKO37M

Expected behavior: rw [hh] should close the first branch. The second branch asking for i < l.length should remain open.

Hence, the second snippet above should have no errors (just a sorry warning).

Actual behavior: First branch has goal i < l.length, second branch is closed.

Versions

Lean 4.24.0-nightly-2025-08-27

Impact

Add 👍 to issues you consider important. If others are impacted by this issue, please ask them to add 👍 to it.