Hi

I got a bug report for `ResumableFunctions`

and after some analysis the problem appeared to be the boxing of an integer used as an index in a comprehension.

Minimal example showing the boxing of the variable `i`

```
function test0()
a = [[1, 2],[3, 4]]
for i in 1:2
u = [v[i] for v in a]
end
end
```

```
code_warntype(test0,())
Variables:
#self# <optimized out>
#69::##69#70
i::Core.Box
u <optimized out>
#temp#::Int64
a::Array{Array{Int64,1},1}
Body:
begin
a::Array{Array{Int64,1},1} = $(Expr(:invoke, MethodInstance for vect(::Array{Int64,1}, ::Vararg{Array{Int64,1},N} where N), :(Base.vect), :($(Expr(:invoke, MethodInstance for vect(::Int64, ::Vararg{Int64,N} where N), :(Base.vect), 1, 2))), :($(Expr(:invoke, MethodInstance for vect(::Int64, ::Vararg{Int64,N} where N), :(Base.vect), 3, 4))))) # line 3:
SSAValue(5) = (Base.select_value)((Base.sle_int)(1, 2)::Bool, 2, (Base.sub_int)(1, 1)::Int64)::Int64
#temp#::Int64 = 1
5:
unless (Base.not_int)((#temp#::Int64 === (Base.add_int)(SSAValue(5), 1)::Int64)::Bool)::Bool goto 19
i::Core.Box = $(Expr(:new, :(Core.Box)))
SSAValue(6) = #temp#::Int64
SSAValue(7) = (Base.add_int)(#temp#::Int64, 1)::Int64
SSAValue(2) = SSAValue(6)
(Core.setfield!)(i::Core.Box, :contents, SSAValue(2))::Int64
#temp#::Int64 = SSAValue(7) # line 3:
#69::##69#70 = $(Expr(:new, :(Main.##69#70), :(i)))
SSAValue(4) = $(Expr(:new, Base.Generator{Array{Array{Int64,1},1},##69#70}, :(#69), :(a)))
$(Expr(:invoke, MethodInstance for collect(::Base.Generator{Array{Array{Int64,1},1},##69#70}), :(Base.collect), SSAValue(4)))
17:
goto 5
19:
return
end::Void
```

Is this normal? I don’t see why the boxing is needed.

If no `for`

loop is used, there is no boxing and if `i`

is not used as an index in a comprehension, there is also no boxing…