Red squiggly line in VS when diagnostic is suppressed

Version Used:

Microsoft Visual Studio Enterprise 2019 Version 16.11.3 Microsoft.CodeAnalysis Version 3.11.0

Steps to Reproduce:

This solution reproduces the issue: https://github.com/dotnet/roslyn/files/7303368/DUTester.zip It's using TreatWarningsAsErrors, meaning the reported CS8509 would fail the build.

1. At line 14 in Program.cs there is a switch expression which reports a CS8509 warning. The warning is suppressed by another analyzer because it's a discriminated union, where the other analyzer checked that all cases are handled.

Expected Behavior: No squiggly lines on the switch expression

Actual Behavior: Squiggly lines on the switch expression. The build still succeeds, so it appears to be a visualization issue in VS.