[julia-users] constructor & function have different promotion rules?

[FANG Colin]

Sorry if this has been discussed somewhere as I am unable to find the 
relative post.

immutable TT
    x::Float64
    y::Float64
end

function tt(x::Float64, y::Float64)
    x + y

end
tt(1,2) # doesn't work
TT(1,2) # works

What rule applies here for TT(1,2)?