Analyzer: Detect using Task<T> with RequestDelegate

[JamesNK]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

RequestDelegate can be used with methods/lambdas that return a value which is then ignored.

Delegate signature:

aspnetcore/src/Http/Http.Abstractions/src/RequestDelegate.cs

Lines 6 to 11 in 86c7e01

	/// <summary>
	/// A function that can process an HTTP request.
	/// </summary>
	/// <param name="context">The <see cref="HttpContext"/> for the request.</param>
	/// <returns>A task that represents the completion of request processing.</returns>
	public delegate Task RequestDelegate(HttpContext context);

Because Task is the base type of Task<T>, generic variance means it's possible to do this:

Task<string> HelloWorld(HttpContext c) => Task.FromResult("Hello " + c.Request.RouteValues["name"]);

// The compiler prefers RequestDelegate overload over the Delegate overload.
// The return value is never used. Not what the user expects.
endpoints.MapGet("/", HelloWorld);

Example: #39956

Describe the solution you'd like

Write an analyzer that detects using a method or lambda that returns Task<T> with RequestDelegate and warns the user.

Additional context

No response

[martincostello]

Same as #36408?

[JamesNK]

No.

Your analyzer issue is for checking what's passed to Result.Ok (and friends).
This analyzer issue is about using a method that returns Task<T> with RequestDelegate.