Define a bunch of zero_tangents that should just NoTangent

src/tangent_types/abstract_zero.jl

@@ -112,8 +112,6 @@
 
 zero_tangent(x::Number) = zero(x)
 
-zero_tangent(::Type) = NoTangent()
-
 function zero_tangent(x::MutableTangent{P}) where {P}
     zb = backing(zero_tangent(backing(x)))
     return MutableTangent{P}(zb)
@@ -171,10 +169,25 @@
     return y
 end
 
+function zero_tangent(::T) where {K,V,T<:AbstractDict{K,V}}
+    return Tangent{T}(Dict{K,guess_zero_tangent_type(V)}())
+end
+
 # Sad heauristic methods we need because of unassigned values
 guess_zero_tangent_type(::Type{T}) where {T<:Number} = T
 guess_zero_tangent_type(::Type{T}) where {T<:Integer} = typeof(float(zero(T)))
 function guess_zero_tangent_type(::Type{<:Array{T,N}}) where {T,N}
     return Array{guess_zero_tangent_type(T),N}
 end
-guess_zero_tangent_type(T::Type) = Any
+guess_zero_tangent_type(T::Type) = Any
+
+# Stuff that conceptually has its own identity regardless of structual implementation and doesn't have a tangent
+zero_tangent(::Base.AbstractLogger) = NoTangent()
+
+# Prevent zero_tangent going wild on the internals
+zero_tangent(::Type) = NoTangent()
+zero_tangent(::Expr) = NoTangent()
+zero_tangent(::Core.Compiler.AbstractInterpreter) = NoTangent()
+zero_tangent(::Core.Compiler.InstructionStream) = NoTangent()
+zero_tangent(::Core.CodeInfo) = NoTangent()
+zero_tangent(::Core.MethodInstance) = NoTangent()

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

@@ -183,6 +183,18 @@ 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}())
+        )
+        if isdefined(Base, :PersistentDict)
+            @test ==(
+                zero_tangent(Base.PersistentDict(1 => 2.4)),
+                Tangent{Base.PersistentDict{Int,Float64}}(Dict{Int, Float64}())
+            )
+        end
+
+
         # Higher order
         # StructuralTangents are valid tangents for themselves (just like Numbers)
         # and indeed we prefer that, otherwise higher order structural tangents are kinda
@@ -200,6 +212,8 @@ end
         @test iszero(zero_tangent(:abc))
         @test iszero(zero_tangent("abc"))
         @test iszero(zero_tangent(sin))
+
+        @test iszero(zero_tangent(:(1 + 1)))
     end
 
     @testset "undef elements Vector" begin