Termination goal for a wf recursive function involving `cases` lacks necessary hypotheses

Prerequisites

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Description

I have a situation in which a well-founded recursive function produces a decreasing_by goal that lacks necessary information about a cases-based match on the recursion variable. If I replace the cases tactic with a match expression, it works.

This might be related to #5690 in that highlights a difference of cases and match, but the symptoms seem different.

Steps to Reproduce

Consider this example:

inductive P1 : Nat → Type where
  | nil : P1 n
  | p1 : P1 n → P1 (n + 1)

opaque P2 : Nat → Type

/--
error: unsolved goals
case refl.refl.refl.refl
n✝ : Nat
a✝ : P1 n✝
p1 : P1 (n✝ + 1)
p2 : P2 (n✝ + 1)
this : sizeOf a✝ < sizeOf a✝.p1
β : Type
⊢ sizeOf a✝ < sizeOf p1
-/
#guard_msgs in
def f (p1 : P1 ms) (p2 : P2 ms) : Unit := by
  cases hp1 : p1
  · exact ()
  · rename_i p1'
    -- recursive call is decreasing:
    have : sizeOf p1' < sizeOf p1 := by simp [hp1]; omega
    -- recursive call:
    exact f p1' sorry
termination_by sizeOf p1
decreasing_by
  · subst_eqs

(The example works with structural recursion, but my real-world use case doesn't.)

Expected behavior:

Because p1 = P1.p1 p1', it should be possible to use this fact to close the termination goal.

Actual behavior:

Because p1 is a free variable without any information about its size or its relation to a✝ (which seems to be the name for p1' in the termination goal), it is impossible to finish the termination proof.

Also note that the have :sizeOf p1' < sizeOf p was transformed into a tautology in the termination goal.

Versions

Lean 4.23.0-rc2
Target: x86_64-unknown-linux-gnu

Impact

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Run the minimal reproducer against Lean 4.23.0-rc2 or the latest nightly, focusing on the cases-based well-founded recursive function, termination_by, and decreasing_by goal. Compare it with the reported match-expression variant; done means the generated termination goal preserves the constructor information needed to close the proof.