variable (R A : Type) [One R] [One A]

class OneHom where
  toFun : R → A
  map_one : toFun One.one = One.one

structure Subone where
  mem : R → Prop
  one_mem : mem One.one

structure Subalgebra [OneHom R A] : Type extends Subone A where
  algebraMap_mem : ∀ r : R, mem (OneHom.toFun r)
  one_mem := OneHom.map_one (R := R) (A := A) ▸ algebraMap_mem One.one

set_option pp.mvars false
/--
error: don't know how to synthesize placeholder
context:
R A : Type
inst✝² : One R
inst✝¹ : One A
inst✝ : OneHom R A
⊢ ∀ (r : R), ?_ R A _example (OneHom.toFun r)
---
error: don't know how to synthesize placeholder
context:
R A : Type
inst✝² : One R
inst✝¹ : One A
inst✝ : OneHom R A
⊢ A → Prop
-/
#guard_msgs in
example [OneHom R A] : Subalgebra R A where
  mem := _
  algebraMap_mem := _