define zero_tangent for Tangent

Author: oxinabox

src/ChainRulesCore.jl

@@ -19,9 +19,9 @@ export StructuralTangent, Tangent, MutableTangent, NoTangent, InplaceableThunk,
 include("debug_mode.jl")
 
 include("tangent_types/abstract_tangent.jl")
+include("tangent_types/structural_tangent.jl")
 include("tangent_types/abstract_zero.jl")
 include("tangent_types/thunks.jl")
-include("tangent_types/structural_tangent.jl")
 include("tangent_types/notimplemented.jl")
 
 include("tangent_arithmetic.jl")

src/tangent_types/abstract_zero.jl

@@ -113,6 +113,9 @@ zero_tangent(x::Number) = zero(x)
 
 zero_tangent(::Type) = NoTangent()
 
+zero_tangent(x::Tangent) = ZeroTangent()
+# TODO: zero_tangent(x::MutableTangent)
+
 @generated function zero_tangent(primal)
     fieldcount(primal) == 0 && return NoTangent()  # no tangent space at all, no need for structural zero.
     zfield_exprs = map(fieldnames(primal)) do fname