Structure fails to compile when nested type parameters are defeq

Prerequisites

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

Kernel error from a specific kind of structure declaration.

Context

I was stubbing out some code (a structure S containing two arrays of a type parametrized by S and two separately defined but defeq other types, analogous to A and B below) when I noticed this.

Steps to Reproduce

/- MWE -/
abbrev A := Nat -- the specific type doesn't matter as long as they're the same
abbrev B := Nat

-- can also replace `T` with e.g. `Prod`,
-- and `Option` with e.g. `Array`
structure T (α β : Type)

structure S where
  x : Option (T A S)
  y : Option (T B S)

2. On the current nightly build (nightly-2025-08-13), 4.22.0-rc4, and 4.21.0, a kernel error is produced:

(kernel) declaration type mismatch, 'S._sizeOf_3_eq' has type
  ∀ (x : Option (T B S)), (fun t => S._sizeOf_2 t = sizeOf t) x
but it is expected to have type
  ∀ (x : Option (T B S)), S._sizeOf_3 x = sizeOf x

Expected behavior: [Clear and concise description of what you expect to happen]
I think this should compile? It does if I remove Option, replace T A S with T B S, or insert the values of A and B directly.

Actual behavior: [Clear and concise description of what actually happens]
See above.

Versions

Lean 4.21.0
Target: arm64-apple-darwin23.6.0 macOS
MacOS Sonoma 14.1

Additional Information

N/A

Impact

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Run the minimal self-contained structure example against the listed Lean versions and confirm the generated kernel error. Start from the nested S declaration and the generated _sizeOf_3_eq mismatch; done means the example compiles without this kernel error while preserving the reported behavior distinctions.