type stability of backslash

[StefanKarpinski]

Note that _var8 is Array{Any,1} and the backslash method as a whole has inferred return type Any:

julia> (@code_typed rand(3,3)\rand(3))[1].args[2][2]
19-element Array{Any,1}:
 Any[:A,Array{Float64,2},0]
 Any[:B,Array{Float64,1},0]
 Any[symbol("#s1986"),(Int64,Int64),18]
 Any[symbol("#s1984"),(Int64,Int64),18]
 Any[:m,Int64,18]
 Any[symbol("#s1983"),(Int64,Int64),18]
 Any[:n,Int64,18]
 Any[symbol("#s1985"),Int64,2]
 Any[:_var1,Array{Float64,1},18]
 Any[:_var0,Int64,18]
 Any[:_var4,Int64,18]
 Any[:_var5,Array{Float64,2},18]
 Any[:_var8,Array{Any,1},18]
 Any[:_var2,Int64,18]
 Any[:_var3,Int64,18]
 Any[:_var6,Int64,18]
 Any[:_var7,Int64,18]
 Any[:_var9,Int64,18]
 Any[:_var10,Int64,18]

julia> (@code_typed rand(3,3)\rand(3))[1].args[3]
:(begin  # linalg/dense.jl, line 409:
        _var2 = (top(arraysize))(A::Array{Float64,2},1)::Int64
        _var3 = (top(arraysize))(A::Array{Float64,2},2)::Int64
        #s1985 = 1
        _var6 = _var2::Int64
        _var7 = (top(box))(Int64,(top(add_int))(1,1))::Int64
        m = _var6::Int64
        #s1985 = _var7::Int64
        _var9 = _var3::Int64
        _var10 = (top(box))(Int64,(top(add_int))(2,1))::Int64
        n = _var9::Int64
        #s1985 = _var10::Int64 # line 410:
        unless m::Int64 === n::Int64::Bool goto 3 # line 411:
        unless istril(A::Array{Float64,2})::Bool goto 1 # line 412:
        unless istriu(A::Array{Float64,2})::Bool goto 0
        _var1 = getindex(A::Array{Float64,2},diagind((top(arraysize))(A::Array{Float64,2},1)::Int64,(top(arraysize))(A::Array{Float64,2},2)::Int64,0)::StepRange{Int64,Int64})::Array{Float64,1}
        return $(Expr(:new, Diagonal{Float64}, :(_var1::Array{Float64,1})))::Diagonal{Float64} \ B::Array{Float64,1}::Array{Float64,1}
        0:
        chksquare(A::Array{Float64,2})::Int64
        return ((top(apply_type))(Triangular,Float64,typeof(A::Array{Float64,2})::Type{Array{Float64,2}},:L,false)::Type{_<:Triangular{Float64,Array{Float64,2},UpLo,IsUnit}})(A::Array{Float64,2})::Triangular{T,S<:AbstractArray{T,2},UpLo,IsUnit} \ B::Array{Float64,1}
        1:  # line 414:
        unless istriu(A::Array{Float64,2})::Bool goto 2
        chksquare(A::Array{Float64,2})::Int64
        return ((top(apply_type))(Triangular,Float64,typeof(A::Array{Float64,2})::Type{Array{Float64,2}},:U,false)::Type{_<:Triangular{Float64,Array{Float64,2},UpLo,IsUnit}})(A::Array{Float64,2})::Triangular{T,S<:AbstractArray{T,2},UpLo,IsUnit} \ B::Array{Float64,1}
        2:  # line 415:
        _var8 = (top(ccall))(:jl_alloc_array_1d,$(Expr(:call1, :(top(apply_type)), :Array, Any, 1))::Type{Array{Any,1}},$(Expr(:call1, :(top(tuple)), :Any, :Int))::(Type{Any},Type{Int64}),Array{Any,1},0,2,0)::Array{Any,1}
        _var4 = (top(arraysize))(A::Array{Float64,2},1)::Int64
        _var0 = (top(arraysize))(A::Array{Float64,2},2)::Int64
        _var5 = (top(ccall))(:jl_alloc_array_2d,$(Expr(:call1, :(top(apply_type)), :Array, Float64, 2))::Type{Array{Float64,2}},$(Expr(:call1, :(top(tuple)), :Any, :Int, :Int))::(Type{Any},Type{Int64},Type{Int64}),Array{Float64,2},0,_var4::Int64,0,_var0::Int64,0)::Array{Float64,2}
        return (top(kwcall))(call,1,:pivot,true,lufact!,_var8::Array{Any,1},copy!(_var5::Array{Float64,2},1,A::Array{Float64,2},1,(top(arraylen))(A::Array{Float64,2})::Int64)::Array{Float64,2})::LU{Float64,Array{Float64,2}} \ B::Array{Float64,1}::Array{Float64,1}
        3:  # line 417:
        return (top(kwcall))(call,1,:pivot,Float64 <: BlasFloat::Bool,qrfact,(top(ccall))(:jl_alloc_array_1d,$(Expr(:call1, :(top(apply_type)), :Array, Any, 1))::Type{Array{Any,1}},$(Expr(:call1, :(top(tuple)), :Any, :Int))::(Type{Any},Type{Int64}),Array{Any,1},0,2,0)::Array{Any,1},A::Array{Float64,2})::Union(QRPivoted{Float64},QRCompactWY{Float64}) \ B::Array{Float64,1}
    end)