Lots of ambiguity errors on v0.5

[tlnagy]

Due to JuliaLang/julia#6190, v0.5 throws a MethodError when function calls are ambiguous. This has come up recently in #881 and in #871. This kind of error is everywhere where we have aliased geometries (e.g. histogram, path, etc):

julia> Pkg.installed("Gadfly")
v"0.4.3+"

julia> using Gadfly

julia> plot(x=1:10, y=1:10, Geom.path,
                   Guide.xlabel("foo"), Guide.ylabel("bar"))
ERROR: MethodError: Gadfly.#plot(::Array{Any,1}, ::Gadfly.#plot, ::Gadfly.Geom.#path, ::Gadfly.Guide.XLabel, ::Gadfly.Guide.YLabel) is ambiguous. Candidates:
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...)
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, elements::Union{Array{Gadfly.Layer,1},Union{DataType,Function,Gadfly.Element,Gadfly.Theme}}...)

julia> plot(x=1:10, y=1:10, Geom.histogram,
                   Guide.xlabel("foo"), Guide.ylabel("bar"))
ERROR: MethodError: Gadfly.#plot(::Array{Any,1}, ::Gadfly.#plot, ::Gadfly.Geom.#histogram, ::Gadfly.Guide.XLabel, ::Gadfly.Guide.YLabel) is ambiguous. Candidates:
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...)
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, elements::Union{Array{Gadfly.Layer,1},Union{DataType,Function,Gadfly.Element,Gadfly.Theme}}...)

julia> plot(x=1:10, y=1:10, Geom.step,
                   Guide.xlabel("foo"), Guide.ylabel("bar"))
ERROR: MethodError: Gadfly.#plot(::Array{Any,1}, ::Gadfly.#plot, ::Gadfly.Geom.#step, ::Gadfly.Guide.XLabel, ::Gadfly.Guide.YLabel) is ambiguous. Candidates:
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...)
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, elements::Union{Array{Gadfly.Layer,1},Union{DataType,Function,Gadfly.Element,Gadfly.Theme}}...)

julia> plot(x=1:10, y=1:10, color=1:10, Geom.histogram2d,
                   Guide.xlabel("foo"), Guide.ylabel("bar"))
ERROR: MethodError: Gadfly.#plot(::Array{Any,1}, ::Gadfly.#plot, ::Gadfly.Geom.#histogram2d, ::Gadfly.Guide.XLabel, ::Gadfly.Guide.YLabel) is ambiguous. Candidates:
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...)
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, elements::Union{Array{Gadfly.Layer,1},Union{DataType,Function,Gadfly.Element,Gadfly.Theme}}...)

julia> plot(x=1:10, y=1:10, color=1:10, Geom.density,
                   Guide.xlabel("foo"), Guide.ylabel("bar"))
ERROR: MethodError: Gadfly.#plot(::Array{Any,1}, ::Gadfly.#plot, ::Gadfly.Geom.#density, ::Gadfly.Guide.XLabel, ::Gadfly.Guide.YLabel) is ambiguous. Candidates:
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...)
  (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, elements::Union{Array{Gadfly.Layer,1},Union{DataType,Function,Gadfly.Element,Gadfly.Theme}}...)

Non-aliased geometries work fine. Currently, the only way around this is by changing Geom.histogram to Geom.histogram(), which prevent the ambiguity by returning a Geometry object instead of a Function. This error is due to the untyped a and b in (::Gadfly.#kw##plot)(::Array{Any,1}, ::Gadfly.#plot, f::Function, a, b, elements::Union{DataType,Function,Gadfly.Element,Gadfly.Theme}...) causing issues when dispatching.

[shashi]

Should probably add one of these as test for #883?