NullReferenceException in NullAnalysisVisitor

[dgrunwald]

These are stack traces collected by SharpDevelop's UDC.
The exceptions are not visible to users as both SharpDevelop and MonoDevelop silently suppress crashes in issue providers.

UDC-5201:

   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitEquality(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data) in NullValueAnalysis.cs:line 1265
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitBinaryOperatorExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data) in NullValueAnalysis.cs:line 1186
   at ICSharpCode.NRefactory.CSharp.BinaryOperatorExpression.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in BinaryOperatorExpression.cs:line 103
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitIfElseStatement(IfElseStatement ifElseStatement, VariableStatusInfo data) in NullValueAnalysis.cs:line 822
   at ICSharpCode.NRefactory.CSharp.IfElseStatement.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in IfElseStatement.cs:line 83

UDC-5199:

   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitOtherBinaryExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data) in NullValueAnalysis.cs:line 1198
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitBinaryOperatorExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data) in NullValueAnalysis.cs:line 1188
   at ICSharpCode.NRefactory.CSharp.BinaryOperatorExpression.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in BinaryOperatorExpression.cs:line 103
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitInvocationExpression(InvocationExpression invocationExpression, VariableStatusInfo data) in NullValueAnalysis.cs:line 1422
   at ICSharpCode.NRefactory.CSharp.InvocationExpression.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in InvocationExpression.cs:line 65
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitVariableInitializer(VariableInitializer variableInitializer, VariableStatusInfo data) in NullValueAnalysis.cs:line 809
   at ICSharpCode.NRefactory.CSharp.VariableInitializer.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in VariableInitializer.cs:line 164

UDC-5197:

   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.NullAnalysisVisitor.VisitForeachStatement(ForeachStatement foreachStatement, VariableStatusInfo data) in NullValueAnalysis.cs:line 848
   at ICSharpCode.NRefactory.CSharp.ForeachStatement.AcceptVisitor[T,S](IAstVisitor`2 visitor, T data) in ForeachStatement.cs:line 98
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.Visit(PendingNode nodeInfo) in NullValueAnalysis.cs:line 421
   at ICSharpCode.NRefactory.CSharp.Analysis.NullValueAnalysis.Analyze() in NullValueAnalysis.cs:line 386
   at ICSharpCode.NRefactory.CSharp.Refactoring.ConstantNullCoalescingConditionIssue.GatherVisitor.GetAnalysis(AstNode parentFunction) in ConstantNullCoalescingConditionIssue.cs:line 121

[dgrunwald]

This is a problem with AST visitors in general: when the input AST is invalid, we might end up visiting a null node. This will cause us to return a null reference, due to:

            public override T AcceptVisitor<T> (IAstVisitor<T> visitor)
            {
                return default (T);
            }

This makes it extremely difficult to write visitors that don't throw NullReferenceExceptions all over the place. To fix this, I think we should introduce a VisitNullNode(AstNode) method to the IAstVisitor interfaces.