Make `Accept` methods of IOperations do nothing instead of throw exception.

src/Compilers/CSharp/Portable/BoundTree/Statement.cs

@@ -691,12 +691,12 @@ partial class BoundSwitchSection
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 
@@ -706,12 +706,12 @@ partial class BoundStatementList
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 
@@ -721,12 +721,12 @@ partial class BoundConditionalGoto
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 
@@ -736,12 +736,12 @@ partial class BoundSequencePoint
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 
@@ -751,12 +751,12 @@ partial class BoundSequencePointWithSpan
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 
@@ -766,12 +766,12 @@ partial class BoundStateMachineScope
 
         public override void Accept(OperationVisitor visitor)
         {
-            throw ExceptionUtilities.Unreachable;
+            visitor.VisitNoneOperation(this);
         }
 
         public override TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument)
         {
-            throw ExceptionUtilities.Unreachable;
+            return visitor.VisitNoneOperation(this, argument);
         }
     }
 }

src/Compilers/Core/Portable/Compilation/OperationVisitor.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System.Diagnostics;
+
 namespace Microsoft.CodeAnalysis.Semantics
 {
     /// <summary>
@@ -17,6 +19,11 @@ public virtual void DefaultVisit(IOperation operation)
         {
             // no-op
         }
+
+        internal void VisitNoneOperation(IOperation operation)
+        {
+            // no-op
+        }
         
         public virtual void VisitBlockStatement(IBlockStatement operation)
         {
@@ -398,6 +405,11 @@ public virtual TResult DefaultVisit(IOperation operation, TArgument argument)
             return default(TResult);
         }
 
+        internal TResult VisitNoneOperation(IOperation operation, TArgument argument)
+        {
+            return default(TResult);
+        }
+
         public virtual TResult VisitBlockStatement(IBlockStatement operation, TArgument argument)
         {
             return DefaultVisit(operation, argument);

src/Compilers/VisualBasic/Portable/BoundTree/Statement.vb

@@ -1511,11 +1511,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1525,11 +1525,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1539,11 +1539,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1553,11 +1553,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1567,11 +1567,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1581,11 +1581,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1595,11 +1595,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1609,11 +1609,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1645,11 +1645,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1659,11 +1659,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1673,11 +1673,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1687,11 +1687,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 
@@ -1701,11 +1701,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Function
 
         Public Overrides Sub Accept(visitor As OperationVisitor)
-            Throw ExceptionUtilities.Unreachable
+            visitor.VisitNoneOperation(Me)
         End Sub
 
         Public Overrides Function Accept(Of TArgument, TResult)(visitor As OperationVisitor(Of TArgument, TResult), argument As TArgument) As TResult
-            Throw ExceptionUtilities.Unreachable
+            Return visitor.VisitNoneOperation(Me, argument)
         End Function
     End Class
 End Namespace