grind: normalization of `UInt16.toNat 0` gets in the way
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Description
Description
Goals around UInt16.toNat can sometimes fail because grind normalizes UInt16.toNat 0 too eagerly.
Context
Discussion here:
#general > grind confused by `UInt.toNat`?
Steps to Reproduce
example (b : UInt16) (h : b = 0) : UInt16.toNat b = 0 := by grind [UInt16.toNat_zero]
Expected behavior: grind should be able to prove this
Actual behavior: grind fails
Versions
4.28.0-nightly-2026-01-12 on live.lean-lang.org
Workaround
For the concrete example above, this is a possible workaround, but it feels like this should not be necessary:
@[grind gen]
theorem UInt16.toNat_of_eq_zero (h : b = 0) : UInt16.toNat b = 0 := by subst h; rfl
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the example from the issue with Lean 4.28.0-nightly-2026-01-12 and compare it with the supplied workaround theorem. Trace how grind normalizes UInt16.toNat 0; the work is done when the original example is proved without requiring that workaround.
Written by the indexing model from the issue text.
Assessment
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 55/100