Fix VSTHRD103 false positives for generic task flows

[Andrew Arnott (AArnott)]

VSTHRD103 currently treats an Async-suffixed overload as preferable even when a synchronous generic API is only composing asynchronous work, such as projecting items to tasks for Task.WhenAll.

This change recognizes variance-aware generic type-parameter flow from inputs into task-bearing results. It covers LINQ projections and more general container, array, tuple, and reduced-extension patterns without suppressing APIs that merely consume tasks or independently return them. VSTHRD002 retains ownership when such a method is explicitly configured as blocking.

Tests cover the reported direct and assigned Select forms, broader task-like flow shapes, contravariant consumers, non-generic near-misses, and configured VSTHRD002 behavior.

Fixes #1662

[Copilot]

Pull request overview

This PR updates the VSTHRD103 (and related VSTHRD002) analyzer logic to avoid false positives when a synchronous generic method is simply composing async-compatible values (e.g., projecting items to Task for Task.WhenAll) rather than blocking. It implements variance-aware detection of async-compatible type flow from parameters into returned container shapes and adds targeted regression tests (Fixes #1662).

Changes:

• Suppress VSTHRD103 suggestions when a generic method’s return value carries async-compatible values that flow from its inputs (including projections, containers/arrays/tuples, and reduced extension methods).
• Ensure VSTHRD002 still reports when such a method is explicitly configured as sync-blocking via additional files.
• Add test coverage for the reported Select + Task.WhenAll scenario and several additional flow/non-flow shapes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD103UseAsyncOptionAnalyzerTests.cs	Adds regression tests for Select→Task.WhenAll and broader generic “task-like flow” shapes.
test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD002UseJtfRunAnalyzerTests.cs	Adds coverage to ensure configured sync-blocking methods still report under VSTHRD002.
src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/VSTHRD103UseAsyncOptionAnalyzer.cs	Uses the new flow-detection helper to avoid reporting VSTHRD103 in valid task-composition patterns.
src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/VSTHRD002UseJtfRunAnalyzer.cs	Excludes flow-through methods from being considered “covered by VSTHRD103” so configured blocking behavior remains reported.
src/Microsoft.VisualStudio.Threading.Analyzers.CSharp/CSharpCommonInterest.cs	Introduces ReturnsAsyncCompatibleValuesFromParameters with variance-aware generic flow analysis.

Suppressed comments (1)

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD103UseAsyncOptionAnalyzerTests.cs:2083

• This test method is marked async but has no await, which produces CS1998 and can introduce avoidable warnings in the test compilation. Consider making it a non-async Task method and return Task.CompletedTask, using discards for the locals that are only present to trigger analyzer behavior.

                async Task ProcessAllAsync()
                {
                    IEnumerable<Task> tasks = {|#0:GetTasks|}();
                    {|#1:Run|}(() => Task.CompletedTask);
                    (Task, int) wrappedTask = {|#2:Wrap|}(Task.CompletedTask);

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.