From a3f710e2a35402bb7f7f96f874e6c696dfc911da Mon Sep 17 00:00:00 2001
From: Jishnu Bhattacharya <jishnub.github@gmail.com>
Date: Sun, 31 Mar 2024 07:26:46 +0530
Subject: [PATCH] Copy for `CartesianIndices`/`LinearIndices` need not
 materialize (#53901)
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Currently,
```julia
julia> C = CartesianIndices((1:2, 1:2))
CartesianIndices((1:2, 1:2))

julia> copy(C)
2×2 Matrix{CartesianIndex{2}}:
 CartesianIndex(1, 1)  CartesianIndex(1, 2)
 CartesianIndex(2, 1)  CartesianIndex(2, 2)
```
However, seeing that a `CartesianIndices` is equivalent to an n-D range,
there doesn't seem to be a need to materialize the result. This PR also
ensures that `copy(C)` returns the same type as `C`.

After this PR:
```julia
julia> C = CartesianIndices((1:2, 1:2))
CartesianIndices((1:2, 1:2))

julia> copy(C)
CartesianIndices((1:2, 1:2))
```
Also, a similar change for `LinearIndices` is added.
---
 base/indices.jl          | 1 +
 base/multidimensional.jl | 4 +++-
 test/abstractarray.jl    | 7 +++++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/base/indices.jl b/base/indices.jl
index 9fadff1cdb5de..e629d2d6a03ff 100644
--- a/base/indices.jl
+++ b/base/indices.jl
@@ -565,6 +565,7 @@ function getindex(iter::LinearIndices, i::AbstractRange{<:Integer})
     @boundscheck checkbounds(iter, i)
     @inbounds isa(iter, LinearIndices{1}) ? iter.indices[1][i] : (first(iter):last(iter))[i]
 end
+copy(iter::LinearIndices) = iter
 # More efficient iteration — predominantly for non-vector LinearIndices
 # but one-dimensional LinearIndices must be special-cased to support OffsetArrays
 iterate(iter::LinearIndices{1}, s...) = iterate(axes1(iter.indices[1]), s...)
diff --git a/base/multidimensional.jl b/base/multidimensional.jl
index 04274cd79c1aa..5fb2bf8cad582 100644
--- a/base/multidimensional.jl
+++ b/base/multidimensional.jl
@@ -4,7 +4,7 @@
 module IteratorsMD
     import .Base: eltype, length, size, first, last, in, getindex, setindex!,
                   min, max, zero, oneunit, isless, eachindex,
-                  convert, show, iterate, promote_rule, to_indices
+                  convert, show, iterate, promote_rule, to_indices, copy
 
     import .Base: +, -, *, (:)
     import .Base: simd_outer_range, simd_inner_length, simd_index, setindex
@@ -476,6 +476,8 @@ module IteratorsMD
     @inline in(i::CartesianIndex, r::CartesianIndices) = false
     @inline in(i::CartesianIndex{N}, r::CartesianIndices{N}) where {N} = all(map(in, i.I, r.indices))
 
+    copy(iter::CartesianIndices) = iter
+
     simd_outer_range(iter::CartesianIndices{0}) = iter
     function simd_outer_range(iter::CartesianIndices)
         CartesianIndices(tail(iter.indices))
diff --git a/test/abstractarray.jl b/test/abstractarray.jl
index 48451d43d79c4..a0a6ba6b2229a 100644
--- a/test/abstractarray.jl
+++ b/test/abstractarray.jl
@@ -332,6 +332,13 @@ end
     end
 end
 
+@testset "copy for LinearIndices/CartesianIndices" begin
+    C = CartesianIndices((1:2, 1:4))
+    @test copy(C) === C
+    L = LinearIndices((1:2, 1:4))
+    @test copy(L) === L
+end
+
 # token type on which to dispatch testing methods in order to avoid potential
 # name conflicts elsewhere in the base test suite
 mutable struct TestAbstractArray end