[ChrisRackauckas]

   julia> propertynames(x)
   ()

Nothing is the correct answer there because there are no properties. `x.y` for any y is an error, so that is correct.

If what you're trying to do is instead discover functions which are compatible with a given signature, you can use what is described in the other post:

    julia> ?("hello", 1, 2.0)[TAB]
    broadcast(f, x::Number...) @ Base.Broadcast broadcast.jl:844
    readuntil(filename::AbstractString, args...; kw...) @ Base io.jl:520
    ...
  

which shows all of the dispatches that match an argument set and thus the functions that can be called on it. Generally the IDE completions do a bit nicer display of that then the REPL though.