Make it possible to use factory for classes without public empty constructor

Author: Per Gårdebrink

src/CommandLine/Core/InstanceBuilder.cs

@@ -39,7 +39,7 @@ public static ParserResult<T> Build<T>(
 
             Func<T> makeDefault = () =>
                 typeof(T).IsMutable()
-                    ? factory.MapValueOrDefault(f => f(), Activator.CreateInstance<T>())
+                    ? factory.MapValueOrDefault(f => f(), () => Activator.CreateInstance<T>())
                     : ReflectionHelper.CreateDefaultImmutableInstance<T>(
                         (from p in specProps select p.Specification.ConversionType).ToArray());
 
@@ -128,7 +128,7 @@ public static ParserResult<T> Build<T>(
 
         private static T BuildMutable<T>(Maybe<Func<T>> factory, IEnumerable<SpecificationProperty> specPropsWithValue, List<Error> setPropertyErrors )
         {
-            var mutable = factory.MapValueOrDefault(f => f(), Activator.CreateInstance<T>());
+            var mutable = factory.MapValueOrDefault(f => f(), () => Activator.CreateInstance<T>());
 
             setPropertyErrors.AddRange(
                 mutable.SetProperties(

src/CommandLine/Infrastructure/CSharpx/Maybe.cs

@@ -371,6 +371,14 @@ public static T2 MapValueOrDefault<T1, T2>(this Maybe<T1> maybe, Func<T1, T2> fu
             return maybe.MatchJust(out value1) ? func(value1) : noneValue;
         }
 
+        /// <summary>
+        /// If contains a values executes a mapping function over it, otherwise returns the value from <paramref name="noneValueFactory"/>.
+        /// </summary>
+        public static T2 MapValueOrDefault<T1, T2>(this Maybe<T1> maybe, Func<T1, T2> func, Func<T2> noneValueFactory) {
+            T1 value1;
+            return maybe.MatchJust(out value1) ? func(value1) : noneValueFactory();
+        }
+
         /// <summary>
         /// Returns an empty list when given <see cref="CSharpx.Nothing{T}"/> or a singleton list when given a <see cref="CSharpx.Just{T}"/>.
         /// </summary>