invoke with broadened type paramaters

[timholy]

ColorTypes defines parametric colors, e.g., RGB{UFixed8} and Gray{Float32}. Often when working with colors, one might want to say convert(Gray, c) without worrying about the element type. ColorTypes contains a traits-function, ccolor ("concrete color"), which performs inferrable "concretization" of the type, so that convert(Gray, ::RGB{UFixed8}) -> Gray{UFixed8} and convert(Gray, ::RGB{Float64}) -> Gray{Float64}. Naturally, if the user asks for type that's already concrete, convert(Gray{Float32}, ::RGB{UFixed8}), that's what s/he gets.

Now let's say I want to override this method to make sure that, when T is a Colorant, it's a concrete Colorant. Here's what I tried:

julia> using Colors

julia> import Base: convert

julia> S = TypeVar(:S)
S

julia> @eval function convert{T<:Colorant,n}(::Type{Array{T,n}}, x::BitArray{n})
           # Specify the type concretely, then invoke the general algorithm on
           # the concrete type
           invoke(convert, (Type{Array{$S,n}}, BitArray{n}), Array{ccolor(T,Gray{Bool}),n}, x)
       end
convert (generic function with 602 methods)

julia> b = bitrand(3,4)
3x4 BitArray{2}:
 false   true  false   true
  true   true   true  false
 false  false   true  false

julia> convert(Array{Gray}, b)
ERROR: invoke: argument type error
 in convert(::Type{Array{ColorTypes.Gray{T<:Union{AbstractFloat,Bool,FixedPointNumbers.FixedPoint{T<:Integer,f}}},N}}, ::BitArray{2}) at ./bitarray.jl:294
 in eval(::Module, ::Any) at ./boot.jl:237

It clearly invokes the correct method, but then I get a mismatch in the type.

Is there any reasonable solution?

[JeffBezanson]

This will require #8974. In the meantime you could try some of the workarounds for avoiding invoke discussed in #13123.

[timholy]

Thanks Jeff. I went with a non-exported _convert method as a workaround (see #15810). It is indeed much easier.