Make iterate(::Reverse) use indexing by default (JuliaLang#43110)

Co-authored-by: Jeff Bezanson <[email protected]> Co-authored-by: Jeff Bezanson <[email protected]> Co-authored-by: Jameson Nash <[email protected]>
lazarusA · Nov 20, 2021 · b64f1cd · b64f1cd
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diff --git a/NEWS.md b/NEWS.md
@@ -25,6 +25,7 @@ Language changes
 * `@time` and `@timev` now take an optional description to allow annotating the source of time reports.
  i.e. `@time "Evaluating foo" foo()` ([#42431])
 * New `@showtime` macro to show both the line being evaluated and the `@time` report ([#42431])
+* Iterating an `Iterators.Reverse` now falls back on reversing the eachindex interator, if possible ([#43110]).
 * Unbalanced Unicode bidirectional formatting directives are now disallowed within strings and comments,
  to mitigate the ["trojan source"](https://www.trojansource.codes) vulnerability ([#42918]).
 

diff --git a/base/iterators.jl b/base/iterators.jl
@@ -113,6 +113,15 @@ first(r::Reverse) = last(r.itr) # and the last shall be first
  (A.itr[idx], (state[1], itrs))
 end
 
+# Fallback method of `iterate(::Reverse{T})` which assumes the collection has `getindex(::T) and `reverse(eachindex(::T))`
+# don't propagate inbounds for this just in case
+function iterate(A::Reverse, state=(reverse(eachindex(A.itr)),))
+ y = iterate(state...)
+ y === nothing && return y
+ idx, itrs = y
+ (A.itr[idx], (state[1], itrs))
+end
+
 reverse(R::AbstractRange) = Base.reverse(R) # copying ranges is cheap
 reverse(G::Generator) = Generator(G.f, reverse(G.iter))
 reverse(r::Reverse) = r.itr