Update comment

Author: willtebbutt

src/rulesets/Base/array.jl

@@ -37,10 +37,13 @@ function rrule(
     return Base.vect(X...), vect_pullback
 end
 
-# This rule isn't certain to be correct, if someone has overloaded `Base.vect` for their type
-# in a way that has a non-identity derivative; but that is a very weird thing to do.
-# so we are confident that this is safe.
-#  Zygote and Diffractor fall over completely without this rule.
+# This rule isn't certain to be correct.
+# This rule _is_ correct when elements of `X` pass straight through into the output without
+# modification. e.g. if the output type of `vect` is a `Vector{Any}`, then this should be
+# correct. If, however, the type of some elements of `X` change in this operation in such a
+# way that their cotangents require projection on the reverse pass of AD, this projection
+# won't happen, possibly causing problems later on in the reverse pass.
+# Zygote and Diffractor fall over completely without this rule.
 function rrule(::typeof(Base.vect), X::Vararg{Any,N}) where {N}
     vect_pullback(ȳ) = (NoTangent(), ntuple(n -> ȳ[n], N)...)
     return Base.vect(X...), vect_pullback