Don't show "expression is never of the provided type" warning if the type could not be resolved.

ICSharpCode.NRefactory.CSharp/Refactoring/CodeIssues/ExpressionIsNeverOfProvidedTypeIssue.cs

@@ -61,6 +61,8 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 				var exprType = ctx.Resolve (isExpression.Expression).Type;
 				var providedType = ctx.ResolveType (isExpression.Type);
 
+				if (exprType.Kind == TypeKind.Unknown || providedType.Kind == TypeKind.Unknown)
+					return;
 				if (IsValidReferenceOrBoxingConversion(exprType, providedType))
 					return;
 				

ICSharpCode.NRefactory.Tests/CSharp/CodeIssues/ExpressionIsNeverOfProvidedTypeIssueTests.cs

@@ -270,6 +270,34 @@ sealed class TestClass
 	{
 		if (x is string[]) ;
 	}
+}";
+			Test<ExpressionIsNeverOfProvidedTypeIssue> (input, 0);
+		}
+		
+		[Test]
+		public void UnknownExpression()
+		{
+			var input = @"
+sealed class TestClass
+{
+	void TestMethod ()
+	{
+		if (unknown is string) ;
+	}
+}";
+			Test<ExpressionIsNeverOfProvidedTypeIssue> (input, 0);
+		}
+		
+		[Test]
+		public void UnknownType()
+		{
+			var input = @"
+sealed class TestClass
+{
+	void TestMethod (int x)
+	{
+		if (x is unknown) ;
+	}
 }";
 			Test<ExpressionIsNeverOfProvidedTypeIssue> (input, 0);
 		}