Fix duplicate _start offset in Tensor.Resize

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/Tensor.cs

@@ -1620,7 +1620,7 @@ public static Tensor<T> Resize<T>(Tensor<T> tensor, ReadOnlySpan<nint> lengths)
 
             if (tensor.IsDense)
             {
-                ReadOnlySpan<T> span = MemoryMarshal.CreateSpan(ref Unsafe.Add(ref tensor.AsTensorSpan()._reference, tensor._start), tensor._values.Length - tensor._start);
+                ReadOnlySpan<T> span = MemoryMarshal.CreateSpan(ref tensor.AsReadOnlyTensorSpan()._reference, (int)tensor.FlattenedLength);
                 Span<T> ospan = MemoryMarshal.CreateSpan(ref output.AsTensorSpan()._reference, (int)output.FlattenedLength);
                 if (newSize >= span.Length)
                 {

src/libraries/System.Numerics.Tensors/tests/TensorTests.cs

@@ -1352,6 +1352,45 @@ public static void TensorResizeTests()
             Assert.Equal(0, t1[1, 4]);
         }
 
+        [Fact]
+        public static void TensorResizeWithStartTests()
+        {
+            Tensor<int> t0 = Tensor.Create([1, 2, 3, 4, 5, 6], start: 2, lengths: [4], strides: []);
+            Tensor<int> t1 = Tensor.Resize(t0, [4]);
+            Assert.Equal([4], t1.Lengths);
+            Assert.Equal(3, t1[0]);
+            Assert.Equal(4, t1[1]);
+            Assert.Equal(5, t1[2]);
+            Assert.Equal(6, t1[3]);
+
+            t1 = Tensor.Resize(t0, [6]);
+            Assert.Equal([6], t1.Lengths);
+            Assert.Equal(3, t1[0]);
+            Assert.Equal(4, t1[1]);
+            Assert.Equal(5, t1[2]);
+            Assert.Equal(6, t1[3]);
+            Assert.Equal(0, t1[4]);
+            Assert.Equal(0, t1[5]);
+
+            t1 = Tensor.Resize(t0, [2]);
+            Assert.Equal([2], t1.Lengths);
+            Assert.Equal(3, t1[0]);
+            Assert.Equal(4, t1[1]);
+
+            t1 = Tensor.Resize(t0, [1, 1]);
+            Assert.Equal([1, 1], t1.Lengths);
+            Assert.Equal(3, t1[0, 0]);
+
+            t1 = Tensor.Resize(t0, [2, 3]);
+            Assert.Equal([2, 3], t1.Lengths);
+            Assert.Equal(3, t1[0, 0]);
+            Assert.Equal(4, t1[0, 1]);
+            Assert.Equal(5, t1[0, 2]);
+            Assert.Equal(6, t1[1, 0]);
+            Assert.Equal(0, t1[1, 1]);
+            Assert.Equal(0, t1[1, 2]);
+        }
+
         [Fact]
         public static void TensorSplitTests()
         {