Miscompile leading to segfault

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

Running the following program segfaults

structure P where
  a : Nat
  b : Nat

structure Q where
  p : P
  n : Nat

@[inline] def mkP (n : Nat) : Option P := some ⟨n, n + 1⟩

def combine (n : Nat) : Option Q :=
  (mkP n).attach.map fun x => ⟨x.val, n⟩

def main : IO Unit := do
  match combine 5 with
  | some q => IO.println s!"{q.p.a} {q.p.b}"
  | none => IO.println "none"

Steps to Reproduce

1. Run the program with lake exe test

Expected behavior: It does not segfault.

Actual behavior: It does.

Versions

4.33.0-nightly-2026-07-07

Additional Information

Impact

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Start by running the minimal reproducer with lake exe test on Lean nightly and confirm the segmentation fault. Trace the compiler behavior around the inline mkP, Option.attach.map, and structure construction; done means the program no longer segfaults and produces the expected output.