Julia - #code_warntype issue with ProgressMeter - performance

Question

Using this for loop with using ProgressMeter:
function test()
#showprogress 1 "Computing..." for n in 3:5:20
print()
end
end
I don't understand why #code_warntype test() returns at three lines a type warning and most importantly how to fix it:
%18 = φ (#2 => %12, #3 => %14)::Union{Nothing, Tuple{Int64,Int64}}

Answer 1

This is how for-loops work internally in Julia, there is nothing you need to worry about or fix here, see https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-iteration-1.
In particular, for-loops call iterate on the object you iterate over, and return nothing if the iterator is done, or a tuple with loopvariable, state. For this particular case, iterate of a StepRange{Int,Int} you thus get back Union{Nothing, Tuple{Int,Int}} from the iterate call:
julia> #code_warntype iterate(3:5:20)
Body::Union{Nothing, Tuple{Int64,Int64}}
[...]

Answer 2

You may take side of while-loop. They don't return small unions.
UPD: Doing so can be overworking. #code_native depicts no difference between implementations. Instead of outer loops, i'd focus on costly function calls to be type-stable.
function f()
for i=1:12
end
end
function g()
i=1
while i<13
i+=1
end
end
#code_warntype f()
Variables
#self#::Core.Compiler.Const(f, false)
#_2::Union{Nothing, Tuple{Int64,Int64}}
i::Int64
Body::Nothing
1 ─ %1 = (1:12)::Core.Compiler.Const(1:12, false)
│ (#_2 = Base.iterate(%1))
│ %3 = (#_2::Core.Compiler.Const((1, 1), false) ===
nothing)::Core.Compiler.Const(false, false)
│ %4 = Base.not_int(%3)::Core.Compiler.Const(true, false)
└── goto #4 if not %4
2 ┄ %6 = #_2::Tuple{Int64,Int64}::Tuple{Int64,Int64}
│ (i = Core.getfield(%6, 1))
│ %8 = Core.getfield(%6, 2)::Int64
│ (#_2 = Base.iterate(%1, %8))
│ %10 = (#_2 === nothing)::Bool
│ %11 = Base.not_int(%10)::Bool
└── goto #4 if not %11
3 ─ goto #2
4 ┄ return
#code_warntype g()
Variables
#self#::Core.Compiler.Const(g, false)
i::Int64
Body::Nothing
1 ─ (i = 1)
2 ┄ %2 = (i < 13)::Bool
└── goto #4 if not %2
3 ─ (i = i + 1)
└── goto #2
4 ─ return