Could you please apply the fix from https://github.com/dotnet/linker/pull/2900/files here? If I understood that fix, I think it may matter for example in the case where a method invocation is passed as an argument, like:

static void M([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] Type t)
{
    DoSomethingWithMethods(t.GetMethods());
}

Maybe @agocke can confirm.

Should I add a test case associated with this fix?

Yes please! Best would be to confirm that the testcase produces an incorrect result before the fix, and then check that it works with the fix.

Yup, that's exactly what my change fixes. I would agree -- creating a test case, confirming it throws an exception, then applying the fix and confirming it no longer does so is a good idea.

Ah, actually it looks like diagnosticNode is properly null checked below, so it won't throw an exception. Probably just won't work (the type will be ArgumentSyntax instead of InvocationExpressionSyntax).

I believe this won't work in Visual Studio. You need to have [ExportCodefixProvider(LanguageNames.CSharp), Shared] on the class. Opened #2932