filter(x -> gcd(x...) < 2, combinations([1:5; 1:5], 2))
# Filter{Function,Base.Combinations{Array{Int64,1}}}((anonymous function),Base.Combinations{Array{Int64,1}}([1,2,3,4,5,1,2,3,4,5],2))

length(filter(x -> gcd(x...) < 2, combinations([1:5; 1:5], 2)))
# MethodError: `length` has no method matching length(::Filter{Function,Base.Combinations{Array{Int64,1}}})