test and fix for nan epsilon of dual^UInt64(0)

src/dual.jl

@@ -261,6 +261,7 @@ end
 Base.mod(z::Dual, n::Number) = Dual(mod(value(z), n), epsilon(z))
 
 # these two definitions are needed to fix ambiguity warnings
+Base.:^(z::Dual, n::Unsigned) = z^Signed(n)
 Base.:^(z::Dual, n::Integer) = Dual(value(z)^n, epsilon(z)*n*value(z)^(n-1))
 Base.:^(z::Dual, n::Rational) = Dual(value(z)^n, epsilon(z)*n*value(z)^(n-1))
 

test/automatic_differentiation_test.jl

@@ -15,6 +15,9 @@ y = x^3.0
 @test value(y) ≈ 2.0^3
 @test epsilon(y) ≈ 3.0*2^2
 
+y = Dual(2.0, 1)^UInt64(0)
+@test !isnan(epsilon(y))
+
 y = sin(x)+exp(x)
 @test value(y) ≈ sin(2)+exp(2)
 @test epsilon(y) ≈ cos(2)+exp(2)