Fix method ambiguity for Base.TwicePrecision (#715)

* Fix method ambiguity for `Base.TwicePrecision`

* Mark tests as not broken

Author: devmotion

src/dual.jl

@@ -78,6 +78,10 @@ end
 @inline Dual{T,V,N}(x::Number) where {T,V,N} = convert(Dual{T,V,N}, x)
 @inline Dual{T,V}(x) where {T,V} = convert(Dual{T,V}, x)
 
+# Fix method ambiguity issue by adapting the definition in Base to `Dual`s
+Dual{T,V,N}(x::Base.TwicePrecision) where {T,V,N} =
+    (Dual{T,V,N}(x.hi) + Dual{T,V,N}(x.lo))::Dual{T,V,N}
+
 ##############################
 # Utility/Accessor Functions #
 ##############################

test/AllocationsTest.jl

@@ -24,7 +24,7 @@ convert_test_574() = convert(ForwardDiff.Dual{Nothing,ForwardDiff.Dual{Nothing,F
     index = 1
     alloc = @allocated ForwardDiff.seed!(duals, x, index, seeds)
     alloc = @allocated ForwardDiff.seed!(duals, x, index, seeds)
-    if VERSION < v"1.9"
+    if VERSION < v"1.9" || VERSION >= v"1.11"
         @test alloc == 0
     else
         @test_broken alloc == 0
@@ -33,7 +33,7 @@ convert_test_574() = convert(ForwardDiff.Dual{Nothing,ForwardDiff.Dual{Nothing,F
     index = 1
     alloc = @allocated ForwardDiff.seed!(duals, x, index, seed)
     alloc = @allocated ForwardDiff.seed!(duals, x, index, seed)
-    if VERSION < v"1.9"
+    if VERSION < v"1.9" || VERSION >= v"1.11"
         @test alloc == 0
     else
         @test_broken alloc == 0

test/DualTest.jl

@@ -667,4 +667,8 @@ end
     @test ForwardDiff.derivative(float, 1)::Float64 === 1.0
 end
 
+@testset "TwicePrecision" begin
+    @test ForwardDiff.derivative(x -> sum(1 .+ x .* (0:0.1:1)), 1) == 5.5
+end
+
 end # module