Disambiguate conversion.

Author: maleadt

lib/cudadrv/error.jl

@@ -20,6 +20,7 @@ function Base.isequal(a::Optional, b::Optional)
 end
 Base.convert(::Type{Optional{T}}, ::Nothing) where T = Optional{T}()
 Base.convert(::Type{Optional{T}}, x) where T = Optional{T}(convert(T, x))
+Base.convert(::Type{Optional{T}}, x::Optional) where T = convert(Optional{T}, x[])
 
 
 """