Type instability of nested function

I have the following function, and it has a type instability (Core.Box).

In the real code, I use f as an argument of another function which calls f multiple times (e.g. bisection to solve f(x)=0).

How can I make this function type stable?

function test(a)
    b = a
    if a < 0
        b = -a
    end
    f(x) = x + b
    f(10)
end

julia> @code_warntype test(1)
Variables
  #self#::Core.Compiler.Const(test, false)
  a::Int64
  b::Core.Box
  f::var"#f#61"

Body::Any
1 ─       (b = Core.Box())
│         Core.NewvarNode(:(f))
│   %3  = a::Int64
│         Core.setfield!(b, :contents, %3)
│   %5  = (a < 0)::Bool
└──       goto #3 if not %5
2 ─ %7  = -a::Int64
└──       Core.setfield!(b, :contents, %7)
3 ┄       (f = %new(Main.:(var"#f#61"), b))
│   %10 = (f)(10)::Any
└──       return %10

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