False ambiguity with supertype dispatch

[timholy]

The following claims an ambiguity, and while there's a workaround it seems like it shouldn't be necessary:

module Amb

abstract type FixedPoint{T <: Integer, f} <: Real end
struct Normed{T<:Unsigned,f} <: FixedPoint{T,f}
    i::T
end
struct Fixed{T <: Signed,f} <: FixedPoint{T,  f}
    i::T
end

# typealias ShortInts Union{Int8,UInt8,Int16,UInt16}
typealias ShortInts Int8

floattype{T<:ShortInts,f}(::Type{FixedPoint{T,f}}) = Float32
floattype{T,f}(::Type{FixedPoint{T,f}}) = Float64

floattype{F<:FixedPoint}(::Type{F}) = floattype(supertype(F))
# The following doesn't exhibit the ambiguity, but seems unnecessarily awkward
# floattype{T,f,F<:FixedPoint{T,f}}(::Type{F}) = floattype(supertype(T))

end

The commented-out version does not trigger ambiguity detection.

julia> using Base.Test

julia> include("/tmp/false_ambiguity.jl")
Amb

julia> setdiff(detect_ambiguities(Amb, Base, Core),
               detect_ambiguities(Base, Core))
Skipping Base.<|
Skipping Base.<|
1-element Array{Tuple{Method,Method},1}:
 (floattype(::Type{Amb.FixedPoint{T,f}}) where {T, f} in Amb at /tmp/false_ambiguity.jl:15, floattype(::Type{F}) where F<:Amb.FixedPoint in Amb at /tmp/false_ambiguity.jl:17)

julia> Amb.floattype(Amb.Normed{UInt8,5})
ERROR: MethodError: Amb.floattype(::Type{Amb.FixedPoint{UInt8,5}}) is ambiguous. Candidates:
  floattype(::Type{F}) where F<:Amb.FixedPoint in Amb at /tmp/false_ambiguity.jl:17
  floattype(::Type{Amb.FixedPoint{T,f}}) where {T, f} in Amb at /tmp/false_ambiguity.jl:15
Stacktrace:
 [1] floattype(::Type{Amb.Normed{UInt8,5}}) at /tmp/false_ambiguity.jl:17

Some alternative definitions also trigger ambiguity detection:

# Also ambiguous:
floattype{T<:Fixed}(::Type{T}) = floattype(supertype(T))
floattype{T<:Normed}(::Type{T}) = floattype(supertype(T))

Example take from FixedPointNumbers.

[yuyichao]

The first case is dup of #11597 (comment) you can workaround it by adding T<:Integer to the first method.

The second case is dup of #20246

[timholy]

I'll close then. Thanks for the workaround!