simp? unfolds more let definitions than a bare simp call

[sgouezel]

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Description

In some situations, simp? creates a simp only call wrongly unfolding let variables.

Context

Zulip discussion at https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/simp.3F.20adding.20let.20reduction/near/493985664

Steps to Reproduce

Run the following snippet:

theorem foo {α : Type} (c : α → α) (x : α) : c x = x := by
  let d := c
  let e := d
  change e x = x
  simp? [e]
  sorry

Expected behavior:
simp? [e] should suggest simp only [e]

Actual behavior:
simp? [e] suggests simp only [e, d], pulling in d while this is not used in the simp call

Another example of misbehavior (probably due to the same bug) is

theorem foo {α : Type} (c : α → α) (x : α) : c x = x := by
  let d := c
  change d x = x
  -- simp [d]
  have : x = x := by
    simp?
  sorry

where the simp? call unfolds to simp only, unless you uncomment simp [d] two lines above: then simp? unfolds to simp only [d].

Versions

Lean 4.16.0-nightly-2025-01-15

Impact

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[nomeata]

Thanks for the report. So the issue is just™ that the simp argument set suggested by simp? is too large, but otherwise this works as expected? So the impact of this bug is on the low side, or am I missing something?

[sgouezel]

Yes, it's "just" that simp? is suggesting a simpset which is too large. It breaks proofs, though, by unfolding too much if you don't realize what is going on and fix the simpset. Not a deal breaker, but a minor inconvenience for expert users, and can be confusing for newcomers.

[nomeata]

Ah, I see: for a non-terminal simp this is more than just a small nuisance. Got it!