From b50992fff0f35aae215657bf34efcda8d4ce3d35 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Tue, 21 Dec 2021 21:18:41 -0500
Subject: [PATCH 1/7] faster copyto

---
 base/abstractarray.jl | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index af29aee6a74b6..8db3fc53f7e63 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -914,9 +914,18 @@ end
 
 function copyto!(dest::AbstractArray, dstart::Integer, src)
     i = Int(dstart)
-    for x in src
-        dest[i] = x
-        i += 1
+    if haslength(src)
+        checkbounds(dest, i)
+        checkbounds(dest, i + length(src) - 1)
+        for x in src
+            @inbounds dest[i] = x
+            i += 1
+        end
+    else
+        for x in src
+            dest[i] = x
+            i += 1
+        end
     end
     return dest
 end

From 8597c9a578f4ab0dca28a9fb3f1eee0ef9f85758 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Tue, 21 Dec 2021 23:47:10 -0500
Subject: [PATCH 2/7] update to N5N3's suggestion

---
 base/abstractarray.jl | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 8db3fc53f7e63..784b5deaf69d0 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -915,11 +915,11 @@ end
 function copyto!(dest::AbstractArray, dstart::Integer, src)
     i = Int(dstart)
     if haslength(src)
-        checkbounds(dest, i)
         checkbounds(dest, i + length(src) - 1)
-        for x in src
-            @inbounds dest[i] = x
-            i += 1
+        I = eachindex(dest)[i]
+        @inbounds for x in src
+            dest[I] = x
+            I = nextind(dest, I)
         end
     else
         for x in src

From 559bcb69fe0cf0bad4cb53eb0869f93da4817236 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Tue, 21 Dec 2021 23:48:55 -0500
Subject: [PATCH 3/7] method for copyto!(dest, src) too

---
 base/abstractarray.jl | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 784b5deaf69d0..9913618195fa5 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -901,13 +901,23 @@ end
 typeof(function copyto! end).name.max_methods = UInt8(1)
 
 function copyto!(dest::AbstractArray, src)
-    destiter = eachindex(dest)
-    y = iterate(destiter)
-    for x in src
-        y === nothing &&
+    if haslength(src)
+        length(dest) < length(src) &&
             throw(ArgumentError("destination has fewer elements than required"))
-        dest[y[1]] = x
-        y = iterate(destiter, y[2])
+        I = firstindex(dest)
+        @inbounds for x in src
+            dest[I] = x
+            I = nextind(dest, I)
+        end
+    else
+        destiter = eachindex(dest)
+        y = iterate(destiter)
+        for x in src
+            y === nothing &&
+                throw(ArgumentError("destination has fewer elements than required"))
+            dest[y[1]] = x
+            y = iterate(destiter, y[2])
+        end
     end
     return dest
 end

From 05b055d360bd319d77ad8845272b585642b9751a Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Wed, 22 Dec 2021 00:45:35 -0500
Subject: [PATCH 4/7] Revert "update to N5N3's suggestion"

This reverts commit eaaefb1baa55e9bae1663bb3557e5fcd955c7ba6.
---
 base/abstractarray.jl | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 9913618195fa5..28188c4d02707 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -925,11 +925,11 @@ end
 function copyto!(dest::AbstractArray, dstart::Integer, src)
     i = Int(dstart)
     if haslength(src)
+        checkbounds(dest, i)
         checkbounds(dest, i + length(src) - 1)
-        I = eachindex(dest)[i]
-        @inbounds for x in src
-            dest[I] = x
-            I = nextind(dest, I)
+        for x in src
+            @inbounds dest[i] = x
+            i += 1
         end
     else
         for x in src

From 58ad472c9c2ee2eac078396f66787b51a3e89bd2 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Wed, 22 Dec 2021 00:55:18 -0500
Subject: [PATCH 5/7] replace nextind with i+1

---
 base/abstractarray.jl | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 28188c4d02707..496c3c4529ebf 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -904,10 +904,10 @@ function copyto!(dest::AbstractArray, src)
     if haslength(src)
         length(dest) < length(src) &&
             throw(ArgumentError("destination has fewer elements than required"))
-        I = firstindex(dest)
+        i = Int(firstindex(dest))
         @inbounds for x in src
-            dest[I] = x
-            I = nextind(dest, I)
+            dest[i] = x
+            i += 1
         end
     else
         destiter = eachindex(dest)

From 713198206d7c9b86bdef38ad29ca136a1e65c3b1 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Wed, 5 Jan 2022 22:42:38 -0500
Subject: [PATCH 6/7] revert copyto!(dest, src)

---
 base/abstractarray.jl | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 496c3c4529ebf..8db3fc53f7e63 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -901,23 +901,13 @@ end
 typeof(function copyto! end).name.max_methods = UInt8(1)
 
 function copyto!(dest::AbstractArray, src)
-    if haslength(src)
-        length(dest) < length(src) &&
+    destiter = eachindex(dest)
+    y = iterate(destiter)
+    for x in src
+        y === nothing &&
             throw(ArgumentError("destination has fewer elements than required"))
-        i = Int(firstindex(dest))
-        @inbounds for x in src
-            dest[i] = x
-            i += 1
-        end
-    else
-        destiter = eachindex(dest)
-        y = iterate(destiter)
-        for x in src
-            y === nothing &&
-                throw(ArgumentError("destination has fewer elements than required"))
-            dest[y[1]] = x
-            y = iterate(destiter, y[2])
-        end
+        dest[y[1]] = x
+        y = iterate(destiter, y[2])
     end
     return dest
 end

From 1d0bfdb3f1a91f2e8f4bfd2f5b888908a4215061 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Wed, 5 Jan 2022 22:51:34 -0500
Subject: [PATCH 7/7] fix an empty case, add tests

---
 base/abstractarray.jl |  5 ++---
 test/arrayops.jl      | 10 ++++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/base/abstractarray.jl b/base/abstractarray.jl
index 8db3fc53f7e63..c59603376b6b4 100644
--- a/base/abstractarray.jl
+++ b/base/abstractarray.jl
@@ -914,9 +914,8 @@ end
 
 function copyto!(dest::AbstractArray, dstart::Integer, src)
     i = Int(dstart)
-    if haslength(src)
-        checkbounds(dest, i)
-        checkbounds(dest, i + length(src) - 1)
+    if haslength(src) && length(dest) > 0
+        @boundscheck checkbounds(dest, i:(i + length(src) - 1))
         for x in src
             @inbounds dest[i] = x
             i += 1
diff --git a/test/arrayops.jl b/test/arrayops.jl
index b2badb66ce93d..e289f6d87d889 100644
--- a/test/arrayops.jl
+++ b/test/arrayops.jl
@@ -2105,6 +2105,16 @@ end
     @test_throws ArgumentError LinearAlgebra.copy_transpose!(a,2:3,1:3,b,1:5,2:7)
 end
 
+@testset "empty copyto!" begin
+    @test isempty(copyto!(Int[], ()))
+    @test isempty(copyto!(Int[], Int[]))
+    @test copyto!([1,2], ()) == [1,2]
+
+    @test isempty(copyto!(Int[], 1, ()))
+    @test isempty(copyto!(Int[], 1, Int[]))
+    @test copyto!([1,2], 1, ()) == [1,2]
+end
+
 module RetTypeDecl
     using Test
     import Base: +, *, broadcast, convert