DArray: Fix copyto! with an empty index range past the end

src/array/copy.jl

@@ -125,6 +125,11 @@ function darray_copyto!(B::DArray{TB,NB}, A::DArray{TA,NA}, Binds=parentindices(
     Binds_range = ntuple(i->to_range(pad1range(Binds, i)), Nmax)
     Ainds_range = ntuple(i->to_range(pad1range(Ainds, i)), Nmax)
 
+    # Nothing to copy. Returning here also keeps the chunk ranges below from
+    # being derived from an empty element range, whose `start` can sit past the
+    # end of the array (and so outside the chunk grid).
+    any(isempty, Binds_range) && return B
+
     # Global element offsets
     Binds_offset = ntuple(i->Binds_range[i].start-1, Nmax)
     Ainds_offset = ntuple(i->Ainds_range[i].start-1, Nmax)

test/array/copyto.jl

@@ -43,3 +43,20 @@ parts_to_test = map(sz->Blocks(sz...), sizes_to_test)
 @testset "Partitioning=$partB<-$partA ($sz)" for partB in parts_to_test, partA in parts_to_test, sz in sizes_to_test
     test_copyto(sz, partA, partB, Float64)
 end
+
+@testset "empty index ranges" begin
+    # An empty range whose `start` is past the end of the array is still a valid,
+    # no-op copy. The chunk range it implies (`fld1(start):fld1(stop)`) can be
+    # non-empty and point outside the chunk grid, so this used to throw.
+    A = distribute(zeros(16, 16), Blocks(4, 4))
+    B = distribute(ones(16, 16), Blocks(4, 4))
+    reference = collect(A)
+    @testset "rows=$rows" for rows in (1:0, 5:4, 16:15, 17:16, 18:17, 20:19)
+        A[rows, 1] = @view B[rows, 1]
+        @test collect(A) == reference
+    end
+    @testset "cols=$cols" for cols in (1:0, 18:17)
+        A[1, cols] = @view B[1, cols]
+        @test collect(A) == reference
+    end
+end