Unintuitive behavior for iterator of views of a vector

General Usage Performance
2

Looks like map! is only defined for AbstractArray in the third argument.

Because you didn’t define its element type correctly :):

julia> @which eltype(typeof(wv))
eltype(::Type) in Base at abstractarray.jl:204

You need

function Base.eltype(::Type{<:WindowViewer{T,V}}) where {T,V}
    return SubArray{T, 1, V, Tuple{UnitRange{Int64}}, true}
end

julia> @btime inplace_map!(Statistics.var, $var_x, $wv)
  11.026 ms (0 allocations: 0 bytes)

julia> VERSION
v"1.8.2"

I think fixing the eltype takes care of the one allocation.

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