Allow declaring DiagnosticAnalyzerAttributes on abstract classes without causing initialization errors

[Rekkonnect]

Background and Motivation

DiagnosticAnalyzerAttribute is inheritable by deriving classes, meaning that abstract classes can make use of it, however Roslyn will attempt to initialize new instances of abstract classes. This will result in AD0001 warnings cryptically indicating that an instance of an abstract class could not be created.

Usage Examples

[DiagnosticAnalyzer(LanguageNames.CSharp)]
public abstract class BaseAnalyzer : DiagnosticAnalyzer
{
    // some abstracted behavior
}

public sealed class DerivedAnalyzer : BaseAnalyzer
{
}

public sealed class AnotherAnalyzer : BaseAnalyzer
{
}

Roslyn will attempt to initialize an instance of DerivedAnalyzer and another of AnotherAnalyzer, but not BaseAnalyzer, because it's abstract. Both instances of the analyzers will be considered to be analyzers for C#.

Solutions

From what I've been looking at, it's this LINQ expression to blame for this. Maybe an additional check for whether the referened type is abstract would deal with this problem.

Risks

Code that would otherwise throw an exception and cause an AD0001 warning, will no longer do. It's a really sad day for badly written analyzers.