`simpDiscrCtor?` is broken in the presence of non-type parameters

[Rob23oba]

Prerequisites

• 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”)

Description

The compiler fails to identify constructor applications as equal to the applications from a cases when the constructor application contains "relevant" parameters (i.e. not a type former or proof).

Context

Writing algorithms on sorted assoc lists.

Steps to Reproduce

inductive MyType (α : Type) (f : α → α → α) where
  | none
  | some (x : α)

set_option trace.Compiler.result true in
def MyType.merge {f : α → α → α} :
    MyType α f → MyType α f → MyType α f
  | .none, x => x
  | x, .none => x
  | .some a, .some b => .some (f a b)

Expected behavior:
In the resulting compiled code, the some _, none branch simply returns the left value and does not perform any allocations.

Actual behavior:
The some _, none branch compiles to a constructor application guarded behind a shared RC check.

Versions

Lean 4.36.0-nightly-2026-09-16

Impact

Add 👍 to issues you consider important. If others are impacted by this issue, please ask them to add 👍 to it.