ReverseDiff doesn't work for noop f(x)=x

[pkofod]

julia> x_seed = [1.0]
1-element Array{Float64,1}:
 1.0

julia> f(x) = x
f (generic function with 1 method)

julia> gcfg = ReverseDiff.GradientConfig(x_seed)
ReverseDiff.GradientConfig

julia> g! = (x, out) -> ReverseDiff.gradient!(out, f, x, gcfg)
(::#17) (generic function with 1 method)

julia> g!([1.0], [0.0])
1-element Array{Float64,1}:
 0.0

julia> f(x) = 1.0*x
f (generic function with 1 method)

julia> g!([1.0], [0.0])
1-element Array{Float64,1}:
 1.0

julia> f(x) = 3.0*x
f (generic function with 1 method)

julia> g!([1.0], [0.0])
1-element Array{Float64,1}:
 3.0

Granted, not the most interesting function in the world, but there must be something that you're missing here, since it doesn't think the function changes with x.

[jrevels]

Thanks for finding this. The solution here is probably to check if the tape is empty after recording, and if so, differentiate the function using ForwardDiff. Actually, I'm pretty sure this worked before and I made a change that broke it. This definitely should be added to the test suite.