Properly handle default parameter value when the parameter type is a value type (#355)

Fixes #354

Author: metoule

src/DynamicExpresso.Core/Resolution/MethodResolution.cs

@@ -165,7 +165,12 @@ public static bool CheckIfMethodIsApplicableAndPrepareIt(MethodData method, Expr
 				if (parameter.HasDefaultValue)
 				{
 					var parameterType = TypeUtils.GetConcreteTypeForGenericMethod(parameter.ParameterType, promotedArgs, method);
-					promotedArgs.Add(Expression.Constant(parameter.DefaultValue, parameterType));
+
+					var defaultValue = parameter.DefaultValue;
+					if (defaultValue is null && parameterType.IsValueType)
+						defaultValue = Activator.CreateInstance(parameterType);
+
+					promotedArgs.Add(Expression.Constant(defaultValue, parameterType));
 				}
 				else if (ReflectionExtensions.HasParamsArrayType(parameter))
 				{

test/DynamicExpresso.UnitTest/GithubIssues.cs

@@ -706,6 +706,11 @@ public List<int> Add(List<int> list, Func<int, int> transform)
 			{
 				return list.Select(i => transform(i)).ToList();
 			}
+
+			public Guid ReturnsGuid(Guid guid = default)
+			{
+				return guid;
+			}
 		}
 
 		[Test]
@@ -877,6 +882,15 @@ public void GitHub_Issue_341()
 
 			Assert.That(interpreter.Eval<bool>("(int)x == 1"), Is.EqualTo((int)x == 1));
 		}
+
+		[Test]
+		public void GitHub_Issue_354()
+		{
+			var interpreter = new Interpreter();
+			interpreter.SetVariable("b", new Functions());
+
+			Assert.That(interpreter.Eval<Guid>("b.ReturnsGuid()"), Is.EqualTo(Guid.Empty));
+		}
 	}
 
 	internal static class GithubIssuesTestExtensionsMethods