Incorrect ambiguity warning

[timholy]

Perhaps I'm being blind, but I can't see any way in which these two methods are ambiguous:

julia> using FixedPointNumbers

julia> scaleinfo{T}(::Type{T}, img::AbstractArray{T}) = 1
scaleinfo (generic function with 1 method)

julia> scaleinfo{To<:Ufixed,From<:FloatingPoint}(::Type{To}, img::AbstractArray{From}) = 2
Warning: New definition 
    scaleinfo(Type{To<:Ufixed},AbstractArray{From<:FloatingPoint,N}) at none:1
is ambiguous with: 
    scaleinfo(Type{T},AbstractArray{T,N}) at none:1.
To fix, define 
    scaleinfo(Type{_<:Ufixed},AbstractArray{_<:FloatingPoint,N})
before the new definition.
scaleinfo (generic function with 2 methods)

julia> isa(Ufixed8, FloatingPoint)
false

This is on 0.3-rc4. I wonder if it's related in any way to #6383.

[timholy]

Also worth pointing out:

julia> typeintersect(Ufixed, FloatingPoint)
None

[timholy]

More examples:

julia> using Color, FixedPointNumbers

julia> import Color.Fractional

julia> import Base.convert

julia> convert{T<:Fractional, N, CV<:ColorValue}(::Type{Array{T,N}}, A::Array{CV,N}) = throw(DimensionMismatch())
Warning: New definition 
    convert(Type{Array{T<:Union(FixedPoint,FloatingPoint),N}},Array{CV<:ColorValue{T},N}) at none:1
is ambiguous with: 
    convert(Type{Array{T,n}},Array{T,n}) at array.jl:218.
To fix, define 
    convert(Type{Array{_<:Union(FixedPoint,FloatingPoint),N}},Array{_<:ColorValue{T},N})
before the new definition.
convert (generic function with 583 methods)

julia> convert{T<:Fractional, N, CV<:ColorValue}(::Type{Array{T}}, A::Array{CV,N}) = myconvert(Array{T}, A)
Warning: New definition 
    convert(Type{Array{T<:Union(FixedPoint,FloatingPoint),N}},Array{CV<:ColorValue{T},N}) at none:1
is ambiguous with: 
    convert(Type{Array{T,N}},Array{T,n}) at array.jl:217.
To fix, define 
    convert(Type{Array{_<:Union(FixedPoint,FloatingPoint),N}},Array{_<:ColorValue{T},N})
before the new definition.
convert (generic function with 583 methods)

I just spent a fair bit of time trying to figure out whether there is some sequence of method definitions that avoids these warnings, to no avail. Currently my only option seems to be to call it something different from convert.

[timholy]

Closed by #11194