Fix type instability

src/flatten.jl

@@ -79,16 +79,20 @@ function flatten(::Type{T}, x::SparseMatrixCSC) where {T<:Real}
 end
 
 function flatten(::Type{T}, x::Tuple) where {T<:Real}
-    x_vecs_and_backs = map(val -> flatten(T, val), x)
-    x_vecs, x_backs = first.(x_vecs_and_backs), last.(x_vecs_and_backs)
-    lengths = map(length, x_vecs)
-    sz = _cumsum(lengths)
+    vec1, back1 = flatten(T, first(x))
+    vec2, back2 = flatten(T, Base.tail(x))
+    l1 = length(vec1)
+    l2 = length(vec2)
     function unflatten_to_Tuple(v::Vector{T})
-        map(x_backs, lengths, sz) do x_back, l, s
-            return x_back(v[(s - l + 1):s])
-        end
+        return (back1(v[1:l1]), back2(v[l1+1:l1+l2]))
     end
-    return reduce(vcat, x_vecs), unflatten_to_Tuple
+    return vcat(vec1, vec2), unflatten_to_Tuple
+end
+
+function flatten(::Type{T}, x::Tuple{}) where {T<:Real}
+    v = T[]
+    unflatten_to_empty_Tuple(::Vector{T}) = x
+    return v, unflatten_to_empty_Tuple
 end
 
 function flatten(::Type{T}, x::NamedTuple) where {T<:Real}