Add spreadmissings

src/Missings.jl

@@ -215,11 +215,17 @@ end
 function (f::SpreadMissings{F})(xs...; kwargs...) where {F}
     if any(x -> x isa AbstractVector{>:Missing}, xs)
         vecs = Base.filter(x -> x isa AbstractVector, xs)
-    nonmissingmask = fill(true, length(vecs[1]))
-    for v in vecs
-        nonmissingmask .&= .!ismissing(v)
-    end
-    nonmissinginds = findall(nonmissingmask)
+
+        findex = eachindex(first(vecs))
+        @assert all(x -> eachindex(x) == findex, vecs)
+
+
+        nonmissingmask = fill(true, length(vecs[1]))
+        for v in vecs
+            nonmissingmask .&= .!ismissing(v)
+        end
+        nonmissinginds = findall(nonmissingmask)
+
         vecs_counter = 1
         newargs = ntuple(length(xs)) do i
             if xs[i] isa AbstractVector