I have the following issue:

```
import ForwardDiff, DiffRules, IrrationalConstants, SpecialFunctions
∂logerfcx(x) = 2 * (x - inv(SpecialFunctions.erfcx(x)) / IrrationalConstants.sqrtπ)
DiffRules.@define_diffrule SpecialFunctions.logerfcx(x) = :(∂logerfcx($x))
ForwardDiff.derivative(SpecialFunctions.logerfcx, 4)
```

This gives the following error:

ERROR: MethodError: no method matching _logerfcx(::ForwardDiff.Dual{ForwardDiff.Tag{typeof(SpecialFunctions.logerfcx), Int64}, Float64, 1})

Closest candidates are:

_logerfcx(::Union{Float32, Float64, BigFloat}) at ~/.julia/packages/SpecialFunctions/NBIqR/src/erf.jl:552

Stacktrace:

[1] logerfcx(x::ForwardDiff.Dual{ForwardDiff.Tag{typeof(SpecialFunctions.logerfcx), Int64}, Int64, 1})

@ SpecialFunctions ~/.julia/packages/SpecialFunctions/NBIqR/src/erf.jl:550

[2] derivative(f::typeof(SpecialFunctions.logerfcx), x::Int64)

@ ForwardDiff ~/.julia/packages/ForwardDiff/tZ5o1/src/derivative.jl:14

[3] top-level scope

@ REPL[15]:1

Why is ForwardDiff not picking up the custom rule I defined?