stack overflow when generating sizeOf spec

Prerequisites

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

The following code causes a stack overflow.

inductive Tree
  | node (children : List (id Tree))

Adding set_option genSizeOfSpec false stops the stack overflow, so the bug is in the generation of the sizeOf spec.

There are similar types that don't cause a stack overflow but result in an error during the sizeOf spec generation:

inductive Nat' -- type mismatch
  | zero
  | succ (n : id Nat')

inductive Tree₁ -- failed to generate sizeOf theorem for Tree₁.node
  | node (children : List (Unit → id Tree₁))

inductive Tree₂ -- failed to generate sizeOf theorem for Tree₂.node
  | node (children : List ((fun α => α) Unit → Tree₂))

inductive Tree₃ -- failed to generate sizeOf theorem for Tree₃.node
  | node (children : List (id (Unit → Tree₃)))

Versions

Lean 4.22.0-nightly-2025-06-26

Impact

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Reproduce the minimal inductive examples from the issue with and without set_option genSizeOfSpec false, starting with the Tree case. Trace the sizeOf specification generation path and compare the reported failures for Nat' and Tree₁–Tree₃; done means generation terminates without a stack overflow and these examples no longer produce the reported errors.