Returned object from function gets lost

[tshort]

Here is an example where the return object from a function gets lost after it is called. The result ends up being Nothing.

This was found by @bsxfan who also reported that this is similar to #2161 and #2562:

• Matrix-valued DualNumber: coding problems  JuliaDiff/ForwardDiff.jl#5

Here is the shortest example I could come up with. It occurs with v0.1.2 and a just updated Julia.

module DualNumbers
import Base.show, Base.cat, Base.vcat, Base.hcat
export DualNum,dualnum

typealias FloatNum Union(Float64, Vector{Float64})

# This is the main new type declared here. The two components can be scalar, vector or matrix, 
# but must agree in type and shape.
type DualNum{T<:FloatNum} 
  st::T # standard part
  di::T # differential part
end
function dualnum{T<:FloatNum}(s::T,d::T) 
  return DualNum(s,d)  # construction given float types that agree
end

dualnum{T<:FloatNum}(s::T) = dualnum(s,zero(s))

function cat(k::Integer,x::DualNum,y::DualNum)
    ST = cat(k,x.st,y.st)
    DI = cat(k,x.di,y.di)
    D = dualnum(ST,DI)
    @show D
    return D
end

vcat(x::DualNum,y::DualNum) = cat(1,x,y)
hcat(x::DualNum,y::DualNum) = cat(2,x,y)

end # module

# Example
a = DualNumbers.dualnum(1.0)
b = DualNumbers.dualnum(2.0)
d = [a,b]
@show d   # shouldn't be Nothing