Fix ITypeSymbolExtensions.IsOrDerivedFromExceptionType to handle type...

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

Fixes https://github.com/dotnet/roslyn/issues/3254

User scenario: User writes code where they have explicit casts to a type parameter deriving from Exception type or its subtype. We will offer incorrect cast removal fixes to remove these explicit casts. Removing such casts will cause their code to not compile. Even worse, if they apply a FixAll occurrences code fix, then all such casts across their (document/project/solution) will be removed, causing tons of compile errors.

Fix description: IsOrDerivedFromExceptionType was only checking for base types of the given type symbol to detect if the type is an Exception type. We also need to handle type parameters which are constrained by Exception or its subtypes.

Testing: Added regression tests + existing tests.

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;
                 }
             }