Most specific method not called when defined last (missing ambiguity warning)

[nalimilan]

In the following example, the second convert method isn't called even if it's more specific than the first one. Defining it first fixes the problem. This happens in both 0.4.6 and master.

type CategoricalArray{T, N, R <: Integer}
    refs::Array{R, N}
    pool::Array{T, N}
end

import Base.convert

convert{T, N, R}(::Type{CategoricalArray{T, N, R}}, A::CategoricalArray) = 1
convert{T, N, R}(::Type{CategoricalArray{T, N, R}}, A::CategoricalArray{T, N, R}) = 2

x = CategoricalArray([1,2], ["a", "b"]);
convert(typeof(x), x)

The ambiguity warning on 0.4.6 gives a hint about what's happening:

WARNING: New definition 
    convert(Type{Main.CategoricalArray{#T<:Any, #N<:Any, #R<:Any}}, Main.CategoricalArray) at none:1
is ambiguous with: 
    convert(Type{Main.CategoricalArray{#T<:Any, #N<:Any, #R<:Any}}, Main.CategoricalArray{#T<:Any, #N<:Any, #R<:Any}) at none:1.
To fix, define 
    convert(Type{Main.CategoricalArray{#T<:Any, #N<:Any, #R<:Any}}, Main.CategoricalArray{#T<:Any, #N<:Any, _<:Integer})
before the new definition.

So IIUC the constraint R <: Integer, which is implicit in the first method, is the cause of the ambiguity. Indeed, adding that constraint to the second method allows it to be called.

I'm not quite sure why the constraint isn't automatically added to the second method, but at least we should get an ambiguity error instead of the current behaviour.

[yuyichao]

The ambiguity shouldn't be there (dup of #6383).

Which version are you using? I do get the following error on both 0.6.0-dev.577 and 0.5.0-rc4

julia> type CategoricalArray{T, N, R <: Integer}
           refs::Array{R, N}
           pool::Array{T, N}
       end

julia> 

julia> import Base.convert

julia> 

julia> convert{T, N, R}(::Type{CategoricalArray{T, N, R}}, A::CategoricalArray) = 1
convert (generic function with 598 methods)

julia> convert{T, N, R}(::Type{CategoricalArray{T, N, R}}, A::CategoricalArray{T, N, R}) = 2
convert (generic function with 599 methods)

julia> 

julia> x = CategoricalArray([1,2], ["a", "b"]);

julia> convert(typeof(x), x)
ERROR: MethodError: convert(::Type{CategoricalArray{String,1,Int64}}, ::CategoricalArray{String,1,Int64}) is ambiguous. Candidates:
  convert{T,N,R}(::Type{CategoricalArray{T,N,R}}, A::CategoricalArray{T,N,R}) at REPL[4]:1
  convert{T,N,R}(::Type{CategoricalArray{T,N,R}}, A::CategoricalArray) at REPL[3]:1

[nalimilan]

Indeed, I confirm I get an error on master. I had only tried with 0.6.0-dev.456 (2016-09-03 13:57 UTC), so it looks like it's been (semi-)fixed last week.