dsimp simplifies too much

[hargoniX]

Prerequisites

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

Consider:

inductive C : Type where
| C1 (b     : Bool) : C
| C2 (c1 c2 : C)    : C
deriving Inhabited

open C

def id1 (b : Bool) : C := C1 b

def id2 (c : C) : C :=
  match c with
  | C1 b     => id1 b
  | C2 c1 c2 => C2 (id2 c1) (id2 c2)

theorem id2_is_idempotent : id2 (id2 c) ≠ id2 c :=
  match c with
  | C1 b  =>  by
    dsimp only [id2]
    -- HERE , which implies that id2 (id1 b) --> id1 b happened at some point
    sorry
  | C2 _ _ => by
    sorry

At "HERE" the goal is id1 b ≠ id1 b so dsimp did a id2 (id1 b) --> id1 b rewrite at some
point (as confirmed by trace.Meta.Tactic.simp.rewrite), this happens despite dsimp only
being instructed to unfold id2, not id1.

Expected behavior: id2 (id1 b) should not be rewritten to id1, this breaks abstraction without the user intending so.
Actual behavior: id2 (id1 b) get's rewritten to id1 b

Versions

"4.12.0-nightly-2024-10-17"

Impact

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