reverse(Iterators.filter) can be incorrect if predicate is impure

[adienes]

From docstring of reverse

reverse(itr) is an iterator over the
same collection but in the reverse order.

yet consider

julia> mutable struct RollingMax
           m
       end

julia> function (rm::RollingMax)(x)
           if x > rm.m
               rm.m = x
               return true
           else
               return false
           end
       end

julia> arr = rand(10);

julia> Iterators.filter(RollingMax(0), arr) |> collect
5-element Vector{Float64}:
 0.20543016348395104
 0.40777744342276
 0.6118207748963927
 0.6765659450947831
 0.8719846507332757

julia> Iterators.filter(RollingMax(0), arr) |> Iterators.reverse |> collect
2-element Vector{Float64}:
 0.839252025474182
 0.8719846507332757

personally I think reverse(f::Filter) should just be removed; it cannot be implemented correctly without knowing the purity of the predicate. another option is to add a warning in the docs? or maybe a third option is to somehow dispatch reverse only when flt can be proven pure---not sure how this is technically possible though.

[adienes]

could this be labeled with bug and iteration ?

[adienes]

closing as #50497 is now merged