fix an empty case, add tests

Author: mcabbott

base/abstractarray.jl

@@ -904,9 +904,8 @@ 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

test/arrayops.jl

@@ -2122,6 +2122,16 @@ 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