fixes #111

src/tracker/array.jl

@@ -22,6 +22,8 @@ TrackedArray(c::Call, x::A, Δ::A) where A <: AbstractArray =
 
 TrackedArray(x::AbstractArray) = TrackedArray(Call(nothing), x, zeros(x))
 
+Base.eltype(x::Type{<:TrackedArray{T}}) where T <: Real = TrackedReal{T}
+
 Base.show(io::IO, ::Type{TrackedArray{T,N,A}}) where {T,N,A<:AbstractArray{T,N}} =
   print(io, "TrackedArray{…,$A}")
 

test/tracker.jl

@@ -1,5 +1,5 @@
 using Flux.Tracker, Base.Test, NNlib
-using Flux.Tracker: gradcheck
+using Flux.Tracker: TrackedReal, gradcheck
 using NNlib
 
 gradtest(f, xs::AbstractArray...) = gradcheck((xs...) -> sum(sin.(f(xs...))), xs...)
@@ -67,6 +67,14 @@ end
   @test x.grad == [8]
 end
 
+@testset "Fallbacks" begin
+  xs = param([1 2; 3 4])
+  @test similar(xs) isa Matrix{Float64}
+  # Remove this test if we do LowerTriangular properly
+  L = LowerTriangular(xs)
+  @test L*L' isa Matrix{TrackedReal{Float64}}
+end
+
 @test @sprintf("%.2f", sum(param([1,2,3]))) == "6.00"
 
 end #testset