fix: cast inherited MapWith results to derived destinations (#947)

src/Mapster.Tests/WhenImplicitInheritanceMapWithDerivedDestination.cs

@@ -0,0 +1,75 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Mapster.Tests
+{
+    /// <summary>
+    /// https://github.com/MapsterMapper/Mapster/issues/947
+    /// </summary>
+    [TestClass]
+    public class WhenImplicitInheritanceMapWithDerivedDestination
+    {
+        [TestCleanup]
+        public void Cleanup()
+        {
+            TypeAdapterConfig.GlobalSettings.Clear();
+            TypeAdapterConfig.GlobalSettings.AllowImplicitDestinationInheritance = false;
+        }
+
+        [TestMethod]
+        public void Inherited_MapWith_On_Base_Destination_Casts_To_Derived_Destination()
+        {
+            var config = new TypeAdapterConfig();
+            config.AllowImplicitDestinationInheritance = true;
+            config.NewConfig<AnimalDto947, Animal947>()
+                .MapWith(src => src.Type == "Bird"
+                    ? (Animal947)new Bird947 { AnimalValue = src.AnimalValueDto }
+                    : new Dog947 { AnimalValue = src.AnimalValueDto });
+
+            var source = new AnimalDto947 { AnimalValueDto = "Hello", Type = "Dog" };
+
+            var dog = source.Adapt<Dog947>(config);
+
+            dog.ShouldBeOfType<Dog947>();
+            dog.AnimalValue.ShouldBe("Hello");
+        }
+
+        [TestMethod]
+        public void Inherited_MapWith_Works_For_Explicit_Source_Destination_Pair()
+        {
+            var config = new TypeAdapterConfig();
+            config.AllowImplicitDestinationInheritance = true;
+            config.NewConfig<AnimalDto947, Animal947>()
+                .MapWith(src => src.Type == "Bird"
+                    ? (Animal947)new Bird947 { AnimalValue = src.AnimalValueDto }
+                    : new Dog947 { AnimalValue = src.AnimalValueDto });
+
+            var source = new AnimalDto947 { AnimalValueDto = "Hello", Type = "Dog" };
+
+            var dog = source.Adapt<AnimalDto947, Dog947>(config);
+
+            dog.ShouldBeOfType<Dog947>();
+            dog.AnimalValue.ShouldBe("Hello");
+        }
+
+        public abstract class Animal947
+        {
+            public string AnimalValue { get; set; } = null!;
+        }
+
+        public class Dog947 : Animal947
+        {
+        }
+
+        public class Bird947 : Animal947
+        {
+        }
+
+        public class AnimalDto947
+        {
+            public string AnimalValueDto { get; set; } = null!;
+
+            public string Type { get; set; } = null!;
+        }
+    }
+}

src/Mapster/TypeAdapterConfig.cs

@@ -431,14 +431,30 @@ private static LambdaExpression CreateMapExpression(CompileArgument arg)
                 throw new CompileException(arg, new InvalidOperationException("ConverterFactory is not found"));
             try
             {
-                return fn(arg);
+                return AdjustInheritedConverterReturnType(fn(arg), arg);
             }
             catch (Exception ex)
             {
                 throw new CompileException(arg, ex);
             }
         }
 
+        private static LambdaExpression AdjustInheritedConverterReturnType(LambdaExpression lambda, CompileArgument arg)
+        {
+            var destinationType = arg.DestinationType;
+            var returnType = lambda.ReturnType;
+            if (returnType == destinationType)
+                return lambda;
+
+            // MapWith configured on a base destination type returns the base type, but implicit
+            // destination inheritance can compile the converter for a derived destination.
+            if (!returnType.IsAssignableFrom(destinationType))
+                return lambda;
+
+            var body = lambda.Body.To(destinationType, force: true);
+            return Expression.Lambda(body, lambda.Parameters);
+        }
+
         private LambdaExpression CreateDynamicMapExpression(TypeTuple tuple)
         {
             var lambda = CreateMapExpression(tuple, MapType.Map);