Turn into an interface function

Author: mtfishman

src/abstractsparsearrayinterface.jl

@@ -54,6 +54,10 @@ end
   return error("Not implemented.")
 end
 
+@interface ::AbstractArrayInterface function isstored(a::SubArray, I::Int...)
+  return isstored(parent(a), Base.reindex(parentindices(a), I)...)
+end
+
 # TODO: Use `Base.to_indices`?
 isstored(a::AbstractArray, I::CartesianIndex) = isstored(a, Tuple(I)...)
 # TODO: Use `Base.to_indices`?
@@ -102,8 +106,8 @@ end
   SparseArraysBase.storedvalues(::T)
 end
 
-function isstored(a::SubArray, I::Int...)
-  return isstored(parent(a), Base.reindex(parentindices(a), I)...)
+@derive (T=SubArray,) begin
+  SparseArraysBase.isstored(::T, ::Int...)
 end
 
 # TODO: Add `ndims` type parameter, like `Base.Broadcast.AbstractArrayStyle`.