VSTHRD110 False Negative in local functions?

[PaulKlein]

Bug description

An async method containing a synchronous local function doesn't trigger VSTHRD110 if that local function calls unawaited task methods.

Repro steps

        async Task DoOperationAsync()
        {
            // Warning CS4014 as expected
            HelperAsync();

            DoOperationInner();

            return;

            void DoOperationInner()
            {
                // No warning, would expect VSTHRD110?
                HelperAsync();
            }
        }
        
        void DoOperation()
        {
            // Raises VSTHRD110 as expected
            HelperAsync();
        }

        Task HelperAsync()
        {
            // e.g. some helper method doing some async work
            return Task.CompletedTask;
        }

Expected behavior

A VSTHRD110 warning on the call to HelperAsync inside DoOperationInner

Actual behavior

No warning is raised

• Version used:
  Microsoft.VisualStudio.Threading.Analyzers v17.12.19

Additional context

Thank you very much for the work you do on these analysers, they are super helpful at preventing issues in my code!
Cheers

[PaulKlein]

Thanks @AArnott for taking a look at this, and thanks again for the excellent set of analysers here.