Stack overflow during elaboration

Prerequisites

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• Reduce the issue to a minimal, self-contained, reproducible test case.
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Description

From this example, it there seems there could be an invalid level metavariable assignment during elaboration, one that forms a loop.

Context

Reported on Zulip

Steps to Reproduce

namespace CategoryTheory
universe v v₁ v₂ v₃ u u₁ u₂ u₃ w₀ w₁

class Quiver (V : Type u) where Hom : V → V → Sort v
structure Functor (V : Type u₁) [Quiver.{v₁} V] (W : Type u₂) [Quiver.{v₂} W] where
class Cat (obj : Type u) extends Quiver.{v + 1} obj : Type max u (v + 1) where
structure Iso {C : Type u} [Cat.{v} C] (X Y : C) where

def Functor.id (C : Type u₁) [Cat.{v₁} C] : Functor C C where

instance pi' {I : Type v₁} (C : I → Type u₁) [∀ i, Cat.{v₁} (C i)] :
    Cat (∀ i, C i) := sorry

variable {C : Type u₁} [Cat.{v₁} C] {D : Type u₂} [Cat.{v₂} D]
  (F : Functor C C)

instance Functor.category :
    Cat.{max u₁ v₂} (Functor C D) := sorry

#check Iso Functor.id F

Expected behavior: This raises a normal error.

Actual behavior: At #check there is a stack overflow.

Versions

Lean 4.15.0-nightly-2024-11-26
Target: x86_64-unknown-linux-gnuLean 4

Additional Information

I tested this locally too on macOS. There are tens of thousands of stack frames for libleanshared.dylib`lean::instantiate_lmvars_fn::visit(lean::level const&). This is what leads me to guess that there is an invalid universe level assignment somewhere.

Impact

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Run the self-contained example against the reported Lean nightly version and inspect the #check expression where elaboration overflows. Start with the reported instantiate_lmvars_fn::visit stack frames and investigate the suspected looping universe-level metavariable assignment; done means the example produces a normal error instead of a stack overflow.