Parametrised method not specialised correctly?

[dpsanders]

The following definition of + for a Vol type representing an N-dimensional volume should (I believe) call the parametrised + method when the two Ns are the same; however, it calls the catch-all method that is designed for Vol objects with different N (EDIT: i.e. those not covered by the more restricted method definition):

immutable Vol{N,T<:Real}
    value::T   
end

Vol{T}(N, x::T) = Vol{N,T}(x)

import Base.+

+(V1::Vol, V2::Vol) = throw(ArgumentError("Volumes of different dimension cannot be added"))
+{N,T}(V1::Vol{N,T}, V2::Vol{N,T}) = Vol{N,T}(V1.value + V2.value)

V1 = Vol(3, 2)
V2 = Vol(3, 4)

julia> V1 + V2
ERROR: ArgumentError: Volumes of different dimension cannot be added
 in + at none:1

[dpsanders]

It does, however, seemingly work correctly if the catch-all method is replaced by the unwieldy

+{N1,N2,T}(V1::Vol{N1,T}, V2::Vol{N2,T}) = throw(ArgumentError("Volumes of different dimension cannot be added"))

[JeffBezanson]

I believe this is because Vol by itself has T<:Real, but in the parameterized method T is unrestricted. Try

+{N,T<:Real}(V1::Vol{N,T}, V2::Vol{N,T}) = Vol{N,T}(V1.value + V2.value)

[JeffBezanson]

Dup of #6383