Given we require the AbstractDicts to be backed by DIcts we must make that constructable

Author: oxinabox

src/tangent_types/structural_tangent.jl

@@ -340,7 +340,7 @@ function Tangent{P}() where {P<:Tuple}
     return Tangent{P,typeof(backing)}(backing)
 end
 
-function Tangent{P}(d::Dict) where {P<:Dict}
+function Tangent{P}(d::Dict) where {P<:AbstractDict}
     return Tangent{P,typeof(d)}(d)
 end
 

test/tangent_types/abstract_zero.jl

@@ -183,8 +183,15 @@ end
 
         @test zero_tangent((1.0, 2.0)) == Tangent{Tuple{Float64,Float64}}(0.0, 0.0)
 
-        @test zero_tangent(Dict{Int,Float64}(1 => 2.4)) ==
-            Tangent{Dict{Int,Float64}}(Dict{Int,Float64}())
+        @test ==(
+            zero_tangent(Dict{Int, Float64}(1=>2.4)),
+             Tangent{Dict{Int,Float64}}(Dict{Int, Float64}())
+        )
+        @test ==(
+            zero_tangent(Base.PersistentDict(1=>2.4)),
+             Tangent{Base.PersistentDict{Int,Float64}}(Dict{Int, Float64}())
+        )
+
 
         # Higher order
         # StructuralTangents are valid tangents for themselves (just like Numbers)