From 2f87ec0140f60c0609a389adc4e19c3466de9d96 Mon Sep 17 00:00:00 2001
From: Ben Jude <ben.aw.jude@gmail.com>
Date: Sat, 24 Oct 2020 14:29:13 +0800
Subject: [PATCH] cross_system_copy_n: Dont attempt a copy if we're tyring to
 copy 0 elements

Attempting to perform this copy with 0 elements caused a debug assertion when compiling with MSVC in debug mode.

Fixes #1275
---
 testing/vector.cu                                   | 13 +++++++++++++
 .../system/cuda/detail/internal/copy_cross_system.h | 13 +++++++------
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/testing/vector.cu b/testing/vector.cu
index 8154b01c6c..297d426a2f 100644
--- a/testing/vector.cu
+++ b/testing/vector.cu
@@ -644,6 +644,19 @@ DECLARE_VECTOR_UNITTEST(TestVectorReserving)
 
 
 
+template <class Vector>
+void TestVectorUninitialisedCopy(void)
+{
+    thrust::device_vector<int> v;
+    std::vector<int> std_vector;
+
+    v = std_vector;
+
+    ASSERT_EQUAL(v.size(), 0);
+}
+DECLARE_VECTOR_UNITTEST(TestVectorUninitialisedCopy);
+
+
 template <class Vector>
 void TestVectorShrinkToFit(void)
 {
diff --git a/thrust/system/cuda/detail/internal/copy_cross_system.h b/thrust/system/cuda/detail/internal/copy_cross_system.h
index ab3b4e5bb7..e17d99ea45 100644
--- a/thrust/system/cuda/detail/internal/copy_cross_system.h
+++ b/thrust/system/cuda/detail/internal/copy_cross_system.h
@@ -100,12 +100,13 @@ namespace __copy {
 
   {
     typedef typename iterator_traits<InputIt>::value_type InputTy;
-
-    trivial_device_copy(derived_cast(sys1),
-                        derived_cast(sys2),
-                        reinterpret_cast<InputTy*>(thrust::raw_pointer_cast(&*result)),
-                        reinterpret_cast<InputTy const*>(thrust::raw_pointer_cast(&*begin)),
-                        n);
+    if (n > 0) {
+      trivial_device_copy(derived_cast(sys1),
+                          derived_cast(sys2),
+                          reinterpret_cast<InputTy*>(thrust::raw_pointer_cast(&*result)),
+                          reinterpret_cast<InputTy const*>(thrust::raw_pointer_cast(&*begin)),
+                          n);
+    }
 
     return result + n;
   }