Fix issue #637 (crash when using append on an empty tensor)

src/arraymancer/tensor/shapeshifting.nim

@@ -352,7 +352,8 @@ proc append*[T](t: Tensor[T], values: varargs[T]): Tensor[T] {.noinit.} =
     $t.rank & " (use `concat` for higher rank tensors)"
   let result_size = t.size + values.len
   result = newTensorUninit[T](result_size)
-  result[0 ..< t.size] = t
+  if t.size > 0:
+    result[0 ..< t.size] = t
   result[t.size ..< result.size] = values
 
 func squeeze*(t: AnyTensor): AnyTensor {.noinit.}=

tests/tensor/test_shapeshifting.nim

@@ -43,7 +43,7 @@
         let d = a.asContiguous(colMajor, force = true)
         # this test needs `toRawSeq` due to the changed layout. `toFlatSeq` provides the
         # same as for `c` above!
         check: d.toRawSeq == @[7, 8, 2, 4, 1, 0, 3, 6, 4, 1, 2, 3, 8, 6, 2, 6, 6, 0]
 
 
         # # Now test with a non contiguous tensor
@@ -113,6 +113,10 @@
       check: a.append(b) == expected
       check: a.append(b.toTensor()) == expected
 
+      # Test fix for issue #637 (https://github.com/mratsim/Arraymancer/issues/637)
+      let c = newTensor[int](0)
+      check: c.append(1) == [1].toTensor
+
     test "Squeeze":
       block:
         let a = toSeq(1..12).toTensor().reshape(3,1,2,1,1,2)