What does "k for k = 1:3" expands out to be?

I am confused here

julia> sum([1,2,3])
6

julia> sum([k for k = 1:3])
6

julia> sum(k for k = 1:3)
6

But when I expand out “k for k = 1:3” manually it does not work

julia> sum(1,2,3)
ERROR: MethodError: no method matching sum(::Int64, ::Int64, ::Int64)
Closest candidates are:
  sum(::Any, ::Any) at reduce.jl:486
  sum(::Any) at reduce.jl:503
  sum(::Any, ::AbstractArray; dims) at reducedim.jl:653
Stacktrace:
 [1] top-level scope at REPL[42]:1

julia> sum(1.0,2.0,3.0)
ERROR: MethodError: no method matching sum(::Float64, ::Float64, ::Float64)
Closest candidates are:
  sum(::Any, ::Any) at reduce.jl:486
  sum(::Any) at reduce.jl:503
  sum(::Any, ::AbstractArray; dims) at reducedim.jl:653
Stacktrace:
 [1] top-level scope at REPL[43]:1

Why doesn’t sum(1,2,3) give me 6?

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