dotnet watch doesn't restart server on lambda return type change

[denis-kralj-codesignal]

SDK version

9.0.203

Environment

MacOS 15.4
Apple M4

Reproduction

1. Call dotnet new web
2. Call dotnet watch run --non-interactive and open the local server in a browser
3. change the return type of the default / GET handler (e.g. from app.MapGet("/", () => "Hello World!"); to app.MapGet("/", () => 1); and save the file
4. refresh the page

Expectation

Step 3 detects the change as a "rude edit" and restarts the application, step 4 shows the new output in the browser

Reality

Step 3 detects the edit and writes this output to the console:

dotnet watch ⌚ File updated: ./Program.cs
dotnet watch 🔥 [dotnet (net9.0)] Hot reload succeeded.

Step 4 fails and the console gets this error message:

fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
      An unhandled exception has occurred while executing the request.
      System.Runtime.CompilerServices.HotReloadException: Attempted to invoke lambda or local function with an unsupported change made while the application is running: /Users/denis/git/dotnet/Program.cs(4,17): error ENC0054: Changing the return type of lambda requires restarting the application.
         at Program.<>c.<<Main>$>b__0_0()
         at lambda_method1(Closure, Object, HttpContext)
         at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Additional info

This behavior was behaving in line with expectations on SDK version 6.0.400, even without the --non-interactive flag

[tmat]

Interesting case.

Roslyn emits new lambda with the new signature and the body of the old lambda to throw HotReloadException. However, since the top-level code did not re-execute, MapGet isn't called and the new lambda isn't registered. So the old lambda is still invoked and throws.

In this change dotnet/roslyn#78009 we introduces a concept of forced restart upon change that has no effect. Since the lambda needs to be recreated and the code that constructs the lambda is in top-level code we should classify this as no-effect change and trigger restart.

[tmat]

Fixed by #51073