Complex valued pow gives stackoverflow error

[GiggleLiu]

julia> using ForwardDiff: Dual

julia> x = Complex(Dual(2, 1.0), Dual(3.0, 0.0))
Dual{Nothing}(2.0,1.0) + Dual{Nothing}(3.0,0.0)*im

julia> x^2
ERROR: StackOverflowError:
Stacktrace:
 [1] Complex at ./complex.jl:12 [inlined] (repeats 2 times)
 [2] float at ./complex.jl:1016 [inlined]
 [3] _cpow(::Complex{Dual{Nothing,Float64,1}}, ::Complex{Dual{Nothing,Float64,1}}) at ./complex.jl:780 (repeats 79984 times)

versioninfo: Julia v1.5.2, ForwardDiff v0.10.12

Also see the error report here: QuantumBFS/Yao.jl#263 (comment)
The second point.

[mcabbott]

The behaviour on ForwardDiff#master, i.e. 0.11-DEV which includes #481, is this:

julia> using ForwardDiff: Dual

julia> x = Complex(Dual(2, 1.0), Dual(3.0, 0.0))
Dual{Nothing}(2.0,1.0) + Dual{Nothing}(3.0,0.0)*im

julia> x^2
Dual{Nothing}(-5.0,4.0) + Dual{Nothing}(12.0,6.0)*im