Unused `arg` in `TapedTask`

[FredericWantiez]

There is a weird edge case when defining a TapedTask with arguments but not using them in the body of the function. For example:

function f(x)
    produce(1)
end

trace = TapedTask(f, 2)
consume(trace)

yields an empty sequence error:

ERROR: LoadError: ArgumentError: reducing over an empty collection is not allowed
Stacktrace:
  [1] _empty_reduce_error()
    @ Base ./reduce.jl:301
  [2] reduce_empty(op::Function, #unused#::Type{Int64})
    @ Base ./reduce.jl:311
  [3] reduce_empty(op::Base.BottomRF{typeof(max)}, #unused#::Type{Int64})
    @ Base ./reduce.jl:330
  [4] reduce_empty_iter
    @ ./reduce.jl:357 [inlined]
  [5] reduce_empty_iter
    @ ./reduce.jl:356 [inlined]
  [6] foldl_impl
    @ ./reduce.jl:49 [inlined]
  [7] mapfoldl_impl
    @ ./reduce.jl:44 [inlined]
  [8] #mapfoldl#244
    @ ./reduce.jl:162 [inlined]
  [9] mapfoldl
    @ ./reduce.jl:162 [inlined]
 [10] #mapreduce#248
    @ ./reduce.jl:289 [inlined]
 [11] mapreduce
    @ ./reduce.jl:289 [inlined]
 [12] #maximum#258
    @ ./reduce.jl:737 [inlined]
 [13] maximum
    @ ./reduce.jl:737 [inlined]
 [14] translate!(tape::Vector{Libtask.AbstractInstruction}, ir::Core.CodeInfo)
    @ Libtask ~/Documents/projects/turing/AdvancedPS.jl/Libtask.jl/src/tapedfunction.jl:320

The same runs fine if the arg is used within the function body:

function f(x)
    produce(x)
end

trace = TapedTask(f, 2)
consume(trace) # 2

The issue seems to be the way the arg_slots are constructed here:

Libtask.jl/src/tapedfunction.jl

Lines 317 to 320 in 8ea2899

	arg_binding_slots = fill(0, maximum(keys(slots)))
	for (k, v) in slots
	arg_binding_slots[k] = v
	end