`error: unknown identifier` in substitution

Prerequisites

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• 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”)

Steps to Reproduce

example {α : Sort u} {x : α} {motive₁ : ∀ y, x = y → Sort v} {motive₂ : ∀ y, x = y → Sort w} (refl : motive₁ x rfl → motive₂ x rfl) {y} h :
  @Eq.rec α x (fun y h => motive₁ y h → motive₂ y h) refl y h = fun z => h ▸ refl (h ▸ z)
:= by cases h; rfl

Expected behavior: Code either succeeds or fails to compute motive.

Actual behavior: error: unknown identifier 'h'

Using the recursor manually succeeds:

example {α : Sort u} {x : α} {motive₁ : ∀ y, x = y → Sort v} {motive₂ : ∀ y, x = y → Sort w} (refl : motive₁ x rfl → motive₂ x rfl) {y} h :
  @Eq.rec α x (fun y h => motive₁ y h → motive₂ y h) refl y h = fun z => h.rec (refl (h.symm.rec (motive := fun x h' => motive₁ x (h.trans h')) z))
:= by cases h; rfl

Versions

Lean 4.23.0-nightly-2025-07-18

Impact

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Run the minimal reproducer from the Steps to Reproduce section against Lean 4.23.0-nightly-2025-07-18, then compare it with the manually written recursor example that succeeds. Investigate the substitution and by cases h; rfl path; done means the original example no longer reports unknown identifier 'h' and the expected computation behavior is preserved.