Don't convert Nullable to Value for a non-type-inferred Return

Fixes issue #188

Author: masonwheeler

src/Boo.Lang.Compiler/Steps/ProcessMethodBodies.cs

@@ -2710,7 +2710,7 @@ override public void LeaveReturnStatement(ReturnStatement node)
 				AssertTypeCompatibility(node.Expression, returnType, expressionType);
 
 			//bind to nullable Value if needed
-			if (TypeSystemServices.IsNullable(expressionType) && !TypeSystemServices.IsNullable(returnType))
+			if (TypeSystemServices.IsNullable(expressionType) && !(TypeSystemServices.IsNullable(returnType) || TypeSystemServices.IsUnknown(returnType)))
 			{
 				// TODO: move to later steps or introduce an implicit conversion operator
 				var mre = new MemberReferenceExpression(node.Expression.LexicalInfo, node.Expression, "Value");

tests/BooCompiler.Tests/GenericsTestFixture.cs

@@ -614,7 +614,13 @@ public void nullable_6()
 		{
 			RunCompilerTestCase(@"nullable-6.boo");
 		}
-		
+
+		[Test]
+		public void nullable_7()
+		{
+			RunCompilerTestCase(@"nullable-7.boo");
+		}
+
 		[Test]
 		public void override_1()
 		{

tests/testcases/net2/generics/nullable-7.boo

@@ -0,0 +1,7 @@
+import System
+
+def NullableTest():
+	return Default(Nullable[of int])
+
+var value = NullableTest()
+assert not value.HasValue