Insufficient method specialization for parametric stagedfunction

[timholy]

In #8504 (comment), I forced method specialization by adding parameters. However, hydra-like, this has caused a new problem: suddenly, the method is no longer being adequately specialized on its splatted inputs.

module AVDebug

type View{T,N,P<:AbstractArray,I<:(RangeIndex...)} <: AbstractArray{T,N}
    parent::P
    indexes::I
    dims::NTuple{N,Int}
end

stagedfunction subview{T,NP}(A::AbstractArray{T,NP}, I::RangeIndex...)
# stagedfunction subview(A::AbstractArray, I::RangeIndex...)
#     T = eltype(A)
    @show I
    N = length(I)
    sizeexprs = [:(length(I[$k])) for k = 1:length(I)]
    It = ntuple(length(I), i->UnitRange{Int})
    Iexprs = [I[k] <: Real ? (:(I[$k]:I[$k])) : (:(I[$k])) for k = 1:length(I)]
    dims = :(tuple($(sizeexprs...)))
    Iext = :(tuple($(Iexprs...)))
    :(View{$T,$N,$A,$It}(A, $Iext, $dims))
end

end

Test file:

import AVDebug

A = reshape(float64(1:300), 5, 5, 3, 4)
S = AVDebug.subview(A, 1:2, 1:2, 1:2, 1:2)
@show typeof(S)
S = AVDebug.subview(A, 1:2, 1:2, 1:2, 2)
@show typeof(S)
S = AVDebug.subview(A, 1:2, 1:2, 2, 1:2)
@show typeof(S)

Output:

julia> include("AVtest.jl")
I = (UnitRange{Int64},UnitRange{Int64},UnitRange{Int64},UnitRange{Int64})
typeof(S) = View{Float64,4,Array{Float64,4},(UnitRange{Int64},UnitRange{Int64},UnitRange{Int64},UnitRange{Int64})}
I = (UnitRange{Int64},UnitRange{Int64},UnitRange{Int64},Int64)
typeof(S) = View{Float64,4,Array{Float64,4},(UnitRange{Int64},UnitRange{Int64},UnitRange{Int64},UnitRange{Int64})}
ERROR: step cannot be zero
 in call at ./range.jl:42
 in - at operators.jl:327
 in colon at range.jl:75
 in anonymous at no file
 in include at ./boot.jl:242
 in include_from_node1 at ./loading.jl:128
while loading /tmp/AVtest.jl, in expression starting on line 8

You can see it never gets into the body of the function.

If I switch to the other function signature (see the commented-out lines), then this test passes (but of course then it starts failing #8504).