Allow constant outputs in ForwardDiff (#526)

* Allow constant outputs in ForwardDiff

* Partials of a constant are a zero-length tuple

* Bump DI to 0.6.4

Author: gdalle

DifferentiationInterface/Project.toml

@@ -1,7 +1,7 @@
 name = "DifferentiationInterface"
 uuid = "a0c0ee7d-e4b9-4e03-894e-1c5f64a51d63"
 authors = ["Guillaume Dalle", "Adrian Hill"]
-version = "0.6.3"
+version = "0.6.4"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"

DifferentiationInterface/ext/DifferentiationInterfaceForwardDiffExt/utils.jl

@@ -28,16 +28,18 @@ function make_dual!(::Type{T}, xdual, x, tx::NTuple{B}) where {T,B}
     return xdual .= Dual{T}.(x, tx...)
 end
 
-myvalue(::Type{T}, ydual::Dual{T}) where {T} = value(T, ydual)
+myvalue(::Type{T}, ydual::Number) where {T} = value(T, ydual)
 myvalue(::Type{T}, ydual) where {T} = myvalue.(T, ydual)
 myvalue!(::Type{T}, y, ydual) where {T} = y .= myvalue.(T, ydual)
 
-myderivative(::Type{T}, ydual::Dual{T}) where {T} = extract_derivative(T, ydual)
+myderivative(::Type{T}, ydual::Number) where {T} = extract_derivative(T, ydual)
 myderivative(::Type{T}, ydual) where {T} = myderivative.(T, ydual)
 myderivative!(::Type{T}, dy, ydual) where {T} = dy .= myderivative.(T, ydual)
 
-function mypartials(::Type{T}, ::Val{B}, ydual::Dual) where {T,B}
-    return partials(T, ydual).values
+function mypartials(::Type{T}, ::Val{B}, ydual::Number) where {T,B}
+    return ntuple(Val(B)) do b
+        partials(T, ydual, b)
+    end
 end
 
 function mypartials(::Type{T}, ::Val{B}, ydual) where {T,B}