Fix ADing Tangent constructor

Author: oxinabox

src/extra_rules.jl

@@ -291,4 +291,12 @@ end
 # https://github.com/JuliaDiff/ChainRulesCore.jl/issues/607
 Base.:(==)(x::Number, ::ZeroTangent) = iszero(x)
 Base.:(==)(::ZeroTangent, x::Number) = iszero(x)
-Base.hash(x::ZeroTangent, h::UInt64) = hash(0, h)
+Base.hash(x::ZeroTangent, h::UInt64) = hash(0, h)
+
+# should this be in ChainRules/ChainRulesCore?
+# Avoid making nested backings, a Tangent is already a valid Tangent for a Tangent, 
+# or a valid second order Tangent for the primal
+function frule((_, ẋ), T::Type{<:Tangent}, x)
+    ẋ::Tangent
+    return T(x), ẋ
+end