Make NoTangent have an eltype of itself.

Author: BioTurboNick

src/tangent_types/abstract_zero.jl

@@ -93,6 +93,8 @@ end
 """
 struct NoTangent <: AbstractZero end
 
+Base.eltype(::Type{NoTangent}) = NoTangent
+
 """
     zero_tangent(primal)
 

test/tangent_types/abstract_zero.jl

@@ -119,6 +119,9 @@
         @test dot(dne, 17.2) == dne
         @test dot(11.9, dne) == dne
 
+        @test eltype(dne) === NoTangent
+        @test eltype(NoTangent) === NoTangent
+
         @test promote_type(NoTangent, Bool) == Bool
         @test promote_type(Bool, NoTangent) == Bool
         @test promote_type(NoTangent, Int64) == Int64