Prerequisites

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• Check that your issue is not already filed:
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• 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
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Description

The unsafe implementation of Option.attach sometimes causes lean to instantly crash. I however suspect that this is a wider problem that would also affect List.attach and others. This is most likely a bug in the compiler where it doesn't expect such a type mismatch (even if α and { x : α // p x } have compatible representations).

Steps to Reproduce

example (x : Nat) : Option Nat := do
  let ⟨a, ha⟩ ← Option.attach (guard (x < 10))
  return 0

Impact

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