fix float(::Dual) and add float(::Type{<:Dual}) (#535)

Author: stevengj

src/dual.jl

@@ -380,8 +380,8 @@ Base.convert(::Type{Dual{T,V,N}}, x) where {T,V,N} = Dual{T}(convert(V, x), zero
 Base.convert(::Type{Dual{T,V,N}}, x::Number) where {T,V,N} = Dual{T}(convert(V, x), zero(Partials{N,V}))
 Base.convert(::Type{D}, d::D) where {D<:Dual} = d
 
-Base.float(d::Dual{T,V,N}) where {T,V,N} = convert(Dual{T,promote_type(V, Float16),N}, d)
-Base.AbstractFloat(d::Dual{T,V,N}) where {T,V,N} = convert(Dual{T,promote_type(V, Float16),N}, d)
+Base.float(::Type{Dual{T,V,N}}) where {T,V,N} = Dual{T,float(V),N}
+Base.float(d::Dual) = convert(float(typeof(d)), d)
 
 ###################################
 # General Mathematical Operations #

test/DualTest.jl

@@ -517,7 +517,7 @@ end
     @test length(UnitRange(Dual(1.5), Dual(3.5))) == 3
     @test length(UnitRange(Dual(1.5,1), Dual(3.5,3))) == 3
 end
-  
+
 if VERSION >= v"1.6.0-rc1"
     @testset "@printf" begin
         for T in (Float16, Float32, Float64, BigFloat)
@@ -528,4 +528,11 @@ if VERSION >= v"1.6.0-rc1"
     end
 end
 
+@testset "float" begin # issue #492
+    @test float(Dual{Nothing, Int, 2}) === Dual{Nothing, Float64, 2}
+    @test float(Dual(1)) isa Dual{Nothing, Float64, 0}
+    @test value.(float.(Dual.(1:4, 2:5, 3:6))) isa Vector{Float64}
+    @test ForwardDiff.derivative(float, 1)::Float64 === 1.0
+end
+
 end # module