convert_single_argument isn't sufficient for vector types

[jakubkaczor]

• are you running newest version (version from docs) ? Yes.
• can you reproduce the bug with a fresh environment ? (]activate --temp; add Makie). Yes.
• What platform + GPU are you on?
  OS: Arch Linux x86_64
  CPU: AMD Ryzen 5 5600U (12) @ 4,29 GHz
  GPU: AMD Radeon Vega Series / Radeon Vega Mobile Series

---

For any type T, I expect implementing convert_single_argument(::T)::S, where S is a type that can be handled by Makie, to be sufficient for T being handled by Makie, but there is a type U and T = Vector{U}, so this is not the case. Here is a minimal example of such type U.

using GLMakie
import GeometryBasics as Geom

# Example of the U type
struct UnitSquare
  origin::Point2
end

Makie.convert_single_argument(ss::Vector{UnitSquare}) = map(ss) do s
    Geom.Rect(s.origin..., 1, 1)
end

square = UnitSquare(Point2(0, 0))
data = [square]
after_conversion = Makie.convert_single_argument(data)
expected_conversion = [Geom.Rect(0, 0, 1, 1)]

# Although the types are the same
@assert typeof(after_conversion) == typeof(expected_conversion)
# This *doesn't* work as expected and poly([square]) fails completely
poly([square])
# although the following works
poly(expected_conversion)

The poly([square]) results in

ERROR: Makie.convert_arguments for the plot type Scatter and its conversion trait PointBased() was unsuccessful.

The signature that could not be converted was:
::Vector{GeometryBasics.HyperRectangle{2, Int64}}

When I implement convert_single_argument for UnitSquare instead of a vector of UnitSquare, the following works as expected.

Makie.convert_single_argument(s::UnitSquare) = Geom.Rect(s.origin..., 1, 1)
poly(square)

[SimonDanisch]

Hm this could be a because the poly recipe does quite a bit of it's own conversion:
https://github.com/MakieOrg/Makie.jl/blob/master/src/basic_recipes/poly.jl#L121

[jakubkaczor]

Oh, I see. Is this something that should be reflected in the documentation, or in the code? I think the latter, but correct me if I am wrong.