Type unstable gradients

[irisallevi]

I copy-pasted this lines from the documentation https://fluxml.ai/Zygote.jl/stable/#Explicit-and-Implicit-Parameters-1

struct Linear
    W
    b
end

(l::Linear)(x) = l.W * x .+ l.b

model = Linear(rand(2, 5), rand(2))
x = rand(5)

Why does the gradient seem type unstable?

@code_warntype gradient(model -> sum(model(x)), model)

MethodInstance for Zygote.gradient(::var"#11#12", ::Linear)
  from gradient(f, args...) in Zygote at [...]
Arguments
  #self#e[36m::Core.Const(Zygote.gradient)e[39m
  fe[36m::Core.Const(var"#11#12"())e[39m
  argse[36m::Tuple{Linear}e[39m
Locals
  @_4e[36m::Int64e[39m
  grade[33me[1m::Union{Nothing, Tuple}e[22me[39m
  backe[91me[1m::Zygote.var"#75#76"e[22me[39m
  ye[91me[1m::Anye[22me[39m
Bodye[33me[1m::Union{Nothing, Tuple{Any}}e[22me[39m
e[90m1 ─e[39m %1  = Core.tuple(f)e[36m::Core.Const((var"#11#12"(),))e[39m
e[90m│  e[39m %2  = Core._apply_iterate(Base.iterate, Zygote.pullback, %1, args)e[91me[1m::Tuple{Any, Zygote.var"#75#76"}e[22me[39m
e[90m│  e[39m %3  = Base.indexed_iterate(%2, 1)e[36m::Core.PartialStruct(Tuple{Any, Int64}, Any[Any, Core.Const(2)])e[39m
e[90m│  e[39m       (y = Core.getfield(%3, 1))
e[90m│  e[39m       (@_4 = Core.getfield(%3, 2))
e[90m│  e[39m %6  = Base.indexed_iterate(%2, 2, @_4::Core.Const(2))e[36m::Core.PartialStruct(Tuple{Zygote.var"#75#76", Int64}, Any[Zygote.var"#75#76", Core.Const(3)])e[39m
e[90m│  e[39m       (back = Core.getfield(%6, 1))
e[90m│  e[39m %8  = Zygote.sensitivity(y)e[91me[1m::Anye[22me[39m
e[90m│  e[39m       (grad = (back)(%8))
e[90m│  e[39m %10 = Zygote.isnothing(grad)e[36m::Boole[39m
e[90m└──e[39m       goto #3 if not %10
e[90m2 ─e[39m       return Zygote.nothing
e[90m3 ─e[39m %13 = Zygote.map(Zygote._project, args, grad::Tuple)e[91me[1m::Tuple{Any}e[22me[39m
e[90m└──e[39m       return %13

[ToucheSir]

Closing for the same reason (question is answered on Discourse) as #1476.