Fix Tensor stride computation for single-element (length <= 1) tensors

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

@@ -853,12 +853,22 @@ public static TensorShape Create<T>(T[]? array)
             if (array is not null)
             {
                 int linearLength = array.Length;
+                nint stride = 1;
+
+                TensorFlags flags = TensorFlags.IsDense | TensorFlags.HasAnyDenseDimensions;
+
+                if (linearLength <= 1)
+                {
+                    stride = 0;
+                    flags |= TensorFlags.IsBroadcast;
+                }
+
                 return new TensorShape(
                     flattenedLength: linearLength,
                     linearLength: linearLength,
                     lengths: [linearLength],
-                    strides: [1],
-                    TensorFlags.IsDense | TensorFlags.HasAnyDenseDimensions
+                    strides: [stride],
+                    flags
                 );
             }
             return default;
@@ -908,14 +918,22 @@ public static TensorShape Create<T>(ref readonly T reference, nint linearLength,
         {
             if (!Unsafe.IsNullRef(in reference))
             {
+                nint stride = 1;
+
                 TensorFlags flags = pinned ? TensorFlags.IsPinned : TensorFlags.None;
                 flags |= TensorFlags.IsDense | TensorFlags.HasAnyDenseDimensions;
 
+                if (linearLength <= 1)
+                {
+                    stride = 0;
+                    flags |= TensorFlags.IsBroadcast;
+                }
+
                 return new TensorShape(
                     flattenedLength: linearLength,
                     linearLength: linearLength,
                     lengths: [linearLength],
-                    strides: [1],
+                    strides: [stride],
                     flags
                 );
             }

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

@@ -255,7 +255,7 @@ public static void ReadOnlyTensorSpanArrayConstructorTests()
             Assert.Equal(1, spanInt.Rank);
             Assert.Equal(0, spanInt.Lengths[0]);
             Assert.Equal(0, spanInt.FlattenedLength);
-            Assert.Equal(1, spanInt.Strides[0]);
+            Assert.Equal(0, spanInt.Strides[0]);
             // Make sure it still throws on index 0
             Assert.Throws<IndexOutOfRangeException>(() => {
                 var spanInt = new ReadOnlyTensorSpan<int>(b);

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

@@ -711,7 +711,7 @@ public static void TensorSpanArrayConstructorTests()
             Assert.Equal(1, spanInt.Rank);
             Assert.Equal(0, spanInt.Lengths[0]);
             Assert.Equal(0, spanInt.FlattenedLength);
-            Assert.Equal(1, spanInt.Strides[0]);
+            Assert.Equal(0, spanInt.Strides[0]);
             // Make sure it still throws on index 0
             Assert.Throws<IndexOutOfRangeException>(() => {
                 var spanInt = new TensorSpan<int>(b);

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

@@ -618,6 +618,77 @@ public static void TensorFactoryCreateTests()
             });
         }
 
+        [Fact]
+        public static void TensorCreateSingleElementTests()
+        {
+            // Tensor.Create with a single-element array should have stride 0
+            Tensor<double> src = Tensor.Create([1.0]);
+            Assert.Equal(1, src.Rank);
+            Assert.Equal(1, src.Lengths[0]);
+            Assert.Equal(0, src.Strides[0]);
+            Assert.Equal(1, src.FlattenedLength);
+            Assert.Equal(1.0, src[0]);
+
+            // CreateFromShapeUninitialized without strides should work
+            Tensor<double> dst = Tensor.CreateFromShapeUninitialized<double>(src.Lengths);
+            Assert.Equal(1, dst.Rank);
+            Assert.Equal(1, dst.Lengths[0]);
+            Assert.Equal(0, dst.Strides[0]);
+            Assert.Equal(1, dst.FlattenedLength);
+
+            // CopyTo should succeed
+            src.CopyTo(dst);
+            Assert.Equal(1.0, dst[0]);
+
+            // CreateFromShapeUninitialized with explicit strides should work
+            dst = Tensor.CreateFromShapeUninitialized<double>(src.Lengths, src.Strides);
+            Assert.Equal(1, dst.Rank);
+            Assert.Equal(1, dst.Lengths[0]);
+            Assert.Equal(0, dst.Strides[0]);
+            Assert.Equal(1, dst.FlattenedLength);
+
+            src.CopyTo(dst);
+            Assert.Equal(1.0, dst[0]);
+
+            // CreateFromShape without strides should also work
+            dst = Tensor.CreateFromShape<double>(src.Lengths);
+            Assert.Equal(1, dst.Rank);
+            Assert.Equal(1, dst.Lengths[0]);
+            Assert.Equal(0, dst.Strides[0]);
+            Assert.Equal(1, dst.FlattenedLength);
+
+            src.CopyTo(dst);
+            Assert.Equal(1.0, dst[0]);
+
+            // CreateFromShape with explicit strides should also work
+            dst = Tensor.CreateFromShape<double>(src.Lengths, src.Strides);
+            Assert.Equal(1, dst.Rank);
+            Assert.Equal(1, dst.Lengths[0]);
+            Assert.Equal(0, dst.Strides[0]);
+            Assert.Equal(1, dst.FlattenedLength);
+
+            src.CopyTo(dst);
+            Assert.Equal(1.0, dst[0]);
+
+            // TensorSpan from single-element span should also have stride 0
+            Span<double> span = [42.0];
+            TensorSpan<double> tensorSpan = new TensorSpan<double>(span);
+            Assert.Equal(1, tensorSpan.Rank);
+            Assert.Equal(1, tensorSpan.Lengths[0]);
+            Assert.Equal(0, tensorSpan.Strides[0]);
+            Assert.Equal(1, tensorSpan.FlattenedLength);
+            Assert.Equal(42.0, tensorSpan[0]);
+
+            // ReadOnlyTensorSpan from single-element span should also have stride 0
+            ReadOnlySpan<double> roSpan = [42.0];
+            ReadOnlyTensorSpan<double> roTensorSpan = new ReadOnlyTensorSpan<double>(roSpan);
+            Assert.Equal(1, roTensorSpan.Rank);
+            Assert.Equal(1, roTensorSpan.Lengths[0]);
+            Assert.Equal(0, roTensorSpan.Strides[0]);
+            Assert.Equal(1, roTensorSpan.FlattenedLength);
+            Assert.Equal(42.0, roTensorSpan[0]);
+        }
+
         [Fact]
         public static void TensorCosineSimilarityTests()
         {