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

[Copilot]

Description

Two shortcut TensorShape.Create overloads hardcoded strides: [1] instead of correctly using stride 0 when linearLength <= 1, bypassing the special case that the normal stride computation path handles correctly. This made it impossible to work with single-element tensors:

var src = Tensor.Create([1.0]);          // stride incorrectly set to 1
var dst = Tensor.CreateFromShapeUninitialized<double>(src.Lengths);  // stride correctly 0
src.CopyTo(dst);  // throws: "Destination is too short"

// Passing explicit strides also fails
var dst2 = Tensor.CreateFromShapeUninitialized<double>(src.Lengths, src.Strides);
// throws: "The provided lengths and strides would allow you to access elements outside the provided memory."

Changes

• TensorShape.Create<T>(T[]? array) — Default stride to 1, then set stride to 0 and add IsBroadcast flag when linearLength <= 1. Affects Tensor.Create(T[]), TensorSpan<T>(T[]), ReadOnlyTensorSpan<T>(T[]).
• TensorShape.Create<T>(ref readonly T, nint, bool) — Same fix. Affects TensorSpan<T>(Span<T>) and ReadOnlyTensorSpan<T>(ReadOnlySpan<T>).
• Existing test corrections — Two tests in TensorSpanTests.cs and ReadOnlyTensorSpanTests.cs asserted stride == 1 for empty arrays; corrected to expect 0 (consistent with auto-computed strides).
• Regression test — TensorCreateSingleElementTests covers Create, CreateFromShape, CreateFromShapeUninitialized (with and without explicit strides), CopyTo, and span-based construction for single-element tensors.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Tensor.CreateFromShape and Tensor.CreateFromShapeUninitialized do not work correctly when FlattenedLength == 1</issue_title>
<issue_description>### Description

It is currently not possible to create a tensor with a single value (FlattenedLength == 1), because it results in an empty backing array. Explicitly specifying the strides causes an exception (The provided lengths and strides would allow you to access elements outside the provided memory.), but it should not, as no memory was provided and it should be allocated correctly by the method itself.

Reproduction Steps

To reproduce, compile and run the code for each scenario. Using CreateFromShape instead of CreateFromShapeUninitialized yields the same results.

A (without explicit strides)

var src = Tensor.Create([1.0]);
Console.WriteLine($"src lengths: {String.Join(", ", src.Lengths.ToArray())}; strides: {String.Join(", ", src.Strides.ToArray())}; flattened length: {src.FlattenedLength}");

var dst = Tensor.CreateFromShapeUninitialized<double>(src.Lengths);
Console.WriteLine($"dst lengths: {String.Join(", ", dst.Lengths.ToArray())}; strides: {String.Join(", ", dst.Strides.ToArray())}; flattened length: {src.FlattenedLength}");

src.CopyTo(dst);

B (with explicit strides)

var src = Tensor.Create([1.0]);
Console.WriteLine($"src lengths: {String.Join(", ", src.Lengths.ToArray())}; strides: {String.Join(", ", src.Strides.ToArray())}; flattened length: {src.FlattenedLength}");

var dst = Tensor.CreateFromShapeUninitialized<double>(src.Lengths, src.Strides); // <- notice the difference here
Console.WriteLine($"dst lengths: {String.Join(", ", dst.Lengths.ToArray())}; strides: {String.Join(", ", dst.Strides.ToArray())}; flattened length: {src.FlattenedLength}");

src.CopyTo(dst);

Expected behavior

In both scenarios, the values in src are successfully copied to dst.

Actual behavior

In both scenarios, an ArgumentException is thrown, but for different reasons.

A

Output

src lengths: 1; strides: 1; flattened length: 1
dst lengths: 1; strides: 0; flattened length: 1

Exception

System.ArgumentException: 'Destination is too short. (Parameter 'destination')'

B

Output

src lengths: 1; strides: 1; flattened length: 1

Exception

System.ArgumentException: 'The provided lengths and strides would allow you to access elements outside the provided memory.'

Regression?

No response

Known Workarounds

No response

Configuration

• .NET: 10.0.3
• .NET SDK: 10.0.200-preview.0.26103.119
• OS: Windows 11 Home 25H2 26200.7922
• Architecture: x64

Other information

No response</issue_description>

<agent_instructions>Ensure that Tensor.Create(T[]) correctly initializes the stride to 0 if length is 1. Add a corresponding regression test and check if any other Create APIs have a similar issue where they short circuit the normal strides computation logic and so fail to account for the length == 1 special case.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@tannergooding Looks like this is rather a bug in `Tensor.Create(T[])` where it fails to do `stride = (length > 1) ? length : 0` and only does `stride = length`

Fix is simple, will have it up momentarily.</comment_new>

• Fixes Tensor.CreateFromShape and Tensor.CreateFromShapeUninitialized do not work correctly when FlattenedLength == 1 #125366

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Pull request overview

Fixes an inconsistency in System.Numerics.Tensors where certain TensorShape.Create fast paths hardcoded a stride of 1, bypassing the library’s invariant that dimensions with length <= 1 must have stride == 0 (broadcast semantics). This unblocks creating and copying single-element tensors via Tensor.Create, TensorSpan, and ReadOnlyTensorSpan.

Changes:

• Fix TensorShape.Create(T[]?) and TensorShape.Create(ref readonly T, nint, bool) to set stride = 0 and include IsBroadcast when linearLength <= 1.
• Update existing tests to expect stride == 0 for empty-array tensor spans.
• Add a regression test covering single-element tensor creation, shape-based creation (with/without explicit strides), and CopyTo.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorShape.cs	Aligns shortcut shape creation with the core stride/broadcast invariants for linearLength <= 1.
src/libraries/System.Numerics.Tensors/tests/TensorTests.cs	Adds regression coverage for single-element tensors across Create* APIs and CopyTo.
src/libraries/System.Numerics.Tensors/tests/TensorSpanTests.cs	Corrects empty-array stride expectation to 0 to match computed strides.
src/libraries/System.Numerics.Tensors/tests/ReadOnlyTensorSpanTests.cs	Corrects empty-array stride expectation to 0 to match computed strides.

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