[Resolver] Fixed infinite loop issue when a type inherits from type

parameter.

ICSharpCode.NRefactory.Tests/CSharp/Resolver/ResolveAtLocationTests.cs

@@ -179,5 +179,28 @@ class Foo {
 	} 
 }"));
 		}
+
+		[Test]
+		public void TestBug6758()
+		{
+			var rr = ResolveAtLocation<TypeResolveResult>(
+				@"using System;                                                                   
+
+namespace TestCrash                                                             
+{                                                                               
+        class A                                                                 
+        {                                                                       
+
+        }                                                                       
+
+        class B<T> : T where T: $A                                               
+        {                                                                       
+
+        }                                                                       
+}
+");
+			Assert.AreEqual("A", rr.Type.Name);
+		}
+
 	}
 }

ICSharpCode.NRefactory/TypeSystem/Implementation/BaseTypeCollector.cs

@@ -55,6 +55,8 @@ namespace ICSharpCode.NRefactory.TypeSystem.Implementation
 			// (e.g. C implements I1 and I2, and both interfaces derive from Object)
 			if (!this.Contains(type)) {
 				foreach (IType baseType in type.DirectBaseTypes) {
+					if (type.Kind != TypeKind.TypeParameter && baseType.Kind == TypeKind.TypeParameter)
+						continue;
 					if (SkipImplementedInterfaces && def != null && def.Kind != TypeKind.Interface && def.Kind != TypeKind.TypeParameter) {
 						if (baseType.Kind == TypeKind.Interface) {
 							// skip the interface