Dispatch broken for containers of objects parametrized with TypeVars

[timholy]

Maybe I'm just not seeing it, but to me this seems to be a bug:

abstract Paint{T, N}

abstract AbstractColor{T, N} <: Paint{T, N}
abstract Color{T}            <: AbstractColor{T, 3}
abstract Transparent{C<:AbstractColor,T,N} <: Paint{T,N}

immutable RGB{T<:FloatingPoint} <: Color{T}
    r::T # Red [0,1]
    g::T # Green [0,1]
    b::T # Blue [0,1]
end

colortype{C<:AbstractColor    }(::Type{C})                  = C
colortype{C<:AbstractColor    }(::Type{Transparent{C}})     = C
colortype{C<:AbstractColor,T  }(::Type{Transparent{C,T}})   = C
colortype{C<:AbstractColor,T,N}(::Type{Transparent{C,T,N}}) = C

T = TypeVar(:T)
colortype(RGB{Float32})                     # yields RGB{Float32}
colortype(RGB)                              # yields RGB{T<:FloatingPoint}
colortype(RGB{T})                           # yields RGB{T}
P = Transparent{RGB{Float32},Float32,4}
colortype(P)                                # yields RGB{Float32}
P = Transparent{RGB{T},T,4}

julia> colortype(P)
ERROR: LoadError: MethodError: `colortype` has no method matching colortype(::Type{Transparent{RGB{T},T,4}})
 in include at ./boot.jl:254
 in include_from_node1 at ./loading.jl:264
while loading /tmp/color_trouble2.jl, in expression starting on line 25

Yet

C = TypeVar(:C,AbstractColor)
N = TypeVar(:N)

julia> typeintersect(P, Transparent{C,T,N})
Transparent{RGB{T},T,4}

julia> P <: Transparent{C,T,N}
true

[timholy]

It's actually the 2nd parameter that's causing the problem, and it depends on whether it's a bound or unbound typevar. Reduced case:

abstract AbstractColor{T, N}
abstract Color{T}            <: AbstractColor{T, 3}
abstract Transparent{C<:AbstractColor,T,N}

immutable RGB{T<:FloatingPoint} <: Color{T}
    r::T # Red [0,1]
    g::T # Green [0,1]
    b::T # Blue [0,1]
end

C = TypeVar(:C,AbstractColor,true)
Tu = TypeVar(:T,false)
Tb = TypeVar(:T,true)
N = TypeVar(:N,true)
B = Transparent{C,Tb,N}

julia> P = Transparent{RGB{Tu},Tu,4}
Transparent{RGB{T},T,4}

julia> typeintersect(Type{P}, Type{B})
Union{}

julia> P = Transparent{RGB{Tu},Tb,4}
Transparent{RGB{T},T,4}

julia> typeintersect(Type{P}, Type{B})
Type{Transparent{RGB{T},T,4}}

The key lines seem to be here.

This may give me enough guidance to work around this.

[mbauman]

Sounds like #2552 (comment) and/or #11597. I've actually stopped restricting type parameters in my own code and just enforcing it by what constructors I provide because of wonkiness like this and because I often want "triangular" constraints which aren't expressible in any case.

[timholy]

Yeah, this indeed popped up in a rework of Color.jl, for which I'm trying to design some nice traits to compensate for the lack of triangular dispatch. It specifically arose through introspection on TypeConstructors which arise from typealiases. I got around it by ditching all the typealiases and generating types by metaprogramming.

[timholy]

Rearing its head again. Here's a simple real-world example:

julia> abstract A{T,N}

julia> immutable B{T} <: A{T,3}
           val::T
       end

julia> len{T,N}(::Type{A{T,N}}) = N
len (generic function with 1 method)

julia> super(B)
A{T,3}

julia> len(super(B))
ERROR: MethodError: `len` has no method matching len(::Type{A{T,3}})

This seems to be a consequence of fixing #7062. I have to say, this seems like a worse problem to have than #7062. What do you think, @vtjnash?

[timholy]

Just to show you how devious I am (in case you had any remaining doubts), here's a workaround:

julia> len2{N}(::Type{A{TypeVar(:T),N}}) = N
len2 (generic function with 1 method)

julia> len2(super(B))
3

[vtjnash]

#12596 (comment)

that answer looks correct to me. the TypeVar T is not being matched in the first example, so the method cannot be used. the official way to avoid this is to use a typealias (although constructing the TypeVar manually is pretty similar).

[timholy]

I wonder whether this issue should turn into "further disambiguate the printing of method tables and types so the proper dispatch rules are obvious"?

[JeffBezanson]

closed by #18457