leanprover-community / leanprover-community/mathlib4
Typeclass search doesn't find `Fintype` for SetLike gadgets
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Description
#2279 needed the following instance
-- Porting note: TODO the following instance should follow from a more general principle
@[to_additive]
noncomputable instance [Fintype G] : Fintype (Submonoid.powers x) :=
inferInstanceAs $ Fintype {y // y ∈ Submonoid.powers x}
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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 with the instance added for #2279 and the Fintype (Submonoid.powers x) entry point shown in the issue. Investigate why typeclass search does not derive this through the SetLike subtype and determine the general principle intended by the porting note. Done means the specific workaround is no longer needed and the instance is found correctly.
Written by the indexing model from the issue text.
Assessment
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100