julia> sum(i for i in 1:2, dims=(1,2))
6

julia> sum(i for i in 1:2; dims=(1,2))
ERROR: MethodError: no method matching mapfoldl(::typeof(identity), ::typeof(Base.add_sum), ::Base.Generator{…}; dims::Tuple{…})
This method does not support all of the given keyword arguments (and may not support any).

Closest candidates are:
  mapfoldl(::Any, ::Any, ::Any; init) got unsupported keyword argument "dims"
   @ Base reduce.jl:167

Stacktrace:
 [1] kwerr(::@NamedTuple{…}, ::Function, ::Function, ::Function, ::Base.Generator{…})
   @ Base ./error.jl:175
 [2] mapreduce(f::Function, op::Function, itr::Base.Generator{UnitRange{…}, typeof(identity)}; kw::@Kwargs{dims::Tuple{…}})
   @ Base ./reduce.jl:299
 [3] sum(f::Function, a::Base.Generator{UnitRange{Int64}, typeof(identity)}; kw::@Kwargs{dims::Tuple{Int64, Int64}})
   @ Base ./reduce.jl:524
 [4] sum(a::Base.Generator{UnitRange{Int64}, typeof(identity)}; kw::@Kwargs{dims::Tuple{Int64, Int64}})
   @ Base ./reduce.jl:553
 [5] top-level scope
   @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> [x+1 for x = 1:2]
2-element Vector{Int64}:
 2
 3

julia> [(x, y) for x in 1:3, y in (:a, :b, :c)]
3×3 Matrix{Tuple{Int64, Symbol}}:
 (1, :a)  (1, :b)  (1, :c)
 (2, :a)  (2, :b)  (2, :c)
 (3, :a)  (3, :b)  (3, :c)

generator = (i for i in 1:2, dims in (1,2))
sum(generator)