No longer promote generic types if their arguments aren't compatible (#277)

Author: metoule

src/DynamicExpresso.Core/Parsing/Parser.cs

@@ -2452,7 +2452,7 @@ private static Expression PromoteExpression(Expression expr, Type type, bool exa
 
 			if (type.IsGenericType && !IsNumericType(type))
 			{
-				var genericType = FindAssignableGenericType(expr.Type, type.GetGenericTypeDefinition());
+				var genericType = FindAssignableGenericType(expr.Type, type);
 				if (genericType != null)
 					return Expression.Convert(expr, genericType);
 			}
@@ -2476,6 +2476,11 @@ private static bool IsCompatibleWith(Type source, Type target)
 				return true;
 			}
 
+			if (target.IsGenericParameter)
+			{
+				return true;
+			}
+
 			if (!target.IsValueType)
 			{
 				return target.IsAssignableFrom(source);
@@ -2626,8 +2631,9 @@ private static bool IsWritable(Expression expression)
 		}
 
 		// from http://stackoverflow.com/a/1075059/209727
-		private static Type FindAssignableGenericType(Type givenType, Type genericTypeDefinition)
+		private static Type FindAssignableGenericType(Type givenType, Type constructedGenericType)
 		{
+			var genericTypeDefinition = constructedGenericType.GetGenericTypeDefinition();
 			var interfaceTypes = givenType.GetInterfaces();
 
 			foreach (var it in interfaceTypes)
@@ -2639,7 +2645,16 @@ private static Type FindAssignableGenericType(Type givenType, Type genericTypeDe
 			}
 
 			if (givenType.IsGenericType && givenType.GetGenericTypeDefinition() == genericTypeDefinition)
+			{
+				// the given type has the same generic type of the fully constructed generic type
+				//  => check if the generic arguments are compatible (e.g. Nullable<int> and Nullable<DateTime>: int is not compatible with DateTime)
+				var givenTypeGenericsArgs = givenType.GenericTypeArguments;
+				var constructedGenericsArgs = constructedGenericType.GenericTypeArguments;
+				if (givenTypeGenericsArgs.Zip(constructedGenericsArgs, (g, c) => IsCompatibleWith(g, c)).Any(compatible => !compatible))
+					return null;
+
 				return givenType;
+			}
 
 			var baseType = givenType.BaseType;
 			if (baseType == null)

test/DynamicExpresso.UnitTest/GithubIssues.cs

@@ -719,6 +719,15 @@ public void GitHub_Issue_263(string paramsArguments)
 			var result = interpreter.Eval<int>($"Utils.ParamArrayObjects({paramsArguments})");
 			Assert.AreEqual(4, result);
 		}
+
+		[Test]
+		public void GitHub_Issue_276()
+		{
+			var interpreter = new Interpreter();
+
+			var result = interpreter.Eval<bool>("((int?)5)>((double?)4)");
+			Assert.IsTrue(result);
+		}
 	}
 
 	internal static class GithubIssuesTestExtensionsMethods