Merge pull request #3271 from mavasani/Issue3254

Fix ITypeSymbolExtensions.IsOrDerivedFromExceptionType to handle type parameters constrained on Exception type or its subtype. Fixes #3254 

src/EditorFeatures/CSharpTest/Diagnostics/RemoveUnnecessaryCast/RemoveUnnecessaryCastTests.cs

@@ -3747,6 +3747,46 @@ class C
 class D : C
 {
 
+}");
+        }
+
+        [WorkItem(3254, "https://github.com/dotnet/roslyn/issues/3254")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)]
+        public void DontRemoveCastToTypeParameterWithExceptionContraint()
+        {
+            TestMissing(
+@"using System;
+
+class Program
+{
+    private static void RequiresCondition<TException>(bool condition, string messageOnFalseCondition)
+            where TException : Exception
+    {
+        if (!condition)
+        {
+            throw [|(TException)Activator.CreateInstance(typeof(TException), messageOnFalseCondition)|];
+        }
+    }
+}");
+        }
+
+        [WorkItem(3254, "https://github.com/dotnet/roslyn/issues/3254")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)]
+        public void DontRemoveCastToTypeParameterWithExceptionSubTypeContraint()
+        {
+            TestMissing(
+@"using System;
+
+class Program
+{
+    private static void RequiresCondition<TException>(bool condition, string messageOnFalseCondition)
+            where TException : ArgumentException
+    {
+        if (!condition)
+        {
+            throw [|(TException)Activator.CreateInstance(typeof(TException), messageOnFalseCondition)|];
+        }
+    }
 }");
         }
     }

src/EditorFeatures/VisualBasicTest/Diagnostics/RemoveUnnecessaryCast/RemoveUnnecessaryCastTests.vb

@@ -2655,6 +2655,42 @@ End Class
 
 Class Derived2 : Inherits Base
 End Class
+</File>
+            TestMissing(markup)
+        End Sub
+
+        <WorkItem(3254, "https://github.com/dotnet/roslyn/issues/3254")>
+        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)>
+        Public Sub DontRemoveCastToTypeParameterWithExceptionContraint()
+            Dim markup =
+<File>
+Imports System
+
+Class Program
+    Private Shared Sub RequiresCondition(Of TException As Exception)(condition As Boolean, messageOnFalseCondition As String)
+        If Not condition Then
+            Throw [|DirectCast(Activator.CreateInstance(GetType(TException), messageOnFalseCondition), TException)|]
+        End If
+    End Sub
+End Class
+</File>
+            TestMissing(markup)
+        End Sub
+
+        <WorkItem(3254, "https://github.com/dotnet/roslyn/issues/3254")>
+        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)>
+        Public Sub DontRemoveCastToTypeParameterWithExceptionSubTypeContraint()
+            Dim markup =
+<File>
+Imports System
+
+Class Program
+    Private Shared Sub RequiresCondition(Of TException As ArgumentException)(condition As Boolean, messageOnFalseCondition As String)
+        If Not condition Then
+            Throw [|DirectCast(Activator.CreateInstance(GetType(TException), messageOnFalseCondition), TException)|]
+        End If
+    End Sub
+End Class
 </File>
             TestMissing(markup)
         End Sub

src/Workspaces/Core/Portable/Shared/Extensions/ITypeSymbolExtensions.cs

@@ -777,12 +777,29 @@ public static bool IsOrDerivesFromExceptionType(this ITypeSymbol type, Compilati
         {
             if (type != null)
             {
-                foreach (var baseType in type.GetBaseTypesAndThis())
+                switch(type.Kind)
                 {
-                    if (baseType.Equals(compilation.ExceptionType()))
-                    {
-                        return true;
-                    }
+                    case SymbolKind.NamedType:
+                        foreach (var baseType in type.GetBaseTypesAndThis())
+                        {
+                            if (baseType.Equals(compilation.ExceptionType()))
+                            {
+                                return true;
+                            }
+                        }
+
+                        break;
+
+                    case SymbolKind.TypeParameter:
+                        foreach (var constraint in ((ITypeParameterSymbol)type).ConstraintTypes)
+                        {
+                            if (constraint.IsOrDerivesFromExceptionType(compilation))
+                            {
+                                return true;
+                            }
+                        }
+
+                        break;
                 }
             }