def Lean.mkRecOnViaCasesOn? (declName : Name) : MetaM (Option DefinitionVal)

Defines recOn for declName to be its casesOn, or returns none if casesOn is not built from projections or has not been built yet.

A type eligible for mkCasesOnViaProjs? is neither recursive nor indexed, so its recOn and casesOn have the same type, and rebuilding the projections would just repeat the work.

def Lean.mkRecOn (n : Name) : MetaM Unit