Dispatch on T <: ForwardDiff.Dual

Not doing so leads to this method also being used for non-Dual LArrays, which currently incurs (unnecessary) allocations.

Author: thomvet

src/LabelledArrays.jl

@@ -1,7 +1,7 @@
 module LabelledArrays
 
 using LinearAlgebra, StaticArrays, ArrayInterfaceCore
-import RecursiveArrayTools, PreallocationTools
+import RecursiveArrayTools, PreallocationTools, ForwardDiff
 
 include("slarray.jl")
 include("larray.jl")
@@ -65,7 +65,7 @@ end
 ArrayInterfaceCore.can_setindex(::Type{<:SLArray}) = false
 
 function PreallocationTools.get_tmp(dc::PreallocationTools.DiffCache,
-                                    u::LArray{T, N, D, Syms}) where {T, N, D, Syms}
+                                    u::LArray{T, N, D, Syms}) where {T <: ForwardDiff.Dual, N, D, Syms}
     nelem = div(sizeof(T), sizeof(eltype(dc.dual_du))) * length(dc.du)
     if nelem > length(dc.dual_du)
         PreallocationTools.enlargedualcache!(dc, nelem)