Support mapping of polymorphic collection with abstract parent type

src/AutoMapper/QueryableExtensions/ProjectionBuilder.cs

@@ -64,7 +64,10 @@ public QueryExpressions CreateProjection(in ProjectionRequest request, LetProper
     QueryExpressions CreateProjection(in ProjectionRequest request, LetPropertyMaps letPropertyMaps, TypeMap typeMap, TypeMap[] polymorphicMaps)
     {
         var instanceParameter = Parameter(request.SourceType, "dto" + request.SourceType.Name);
-        var projection = CreateProjectionCore(request, instanceParameter, typeMap, letPropertyMaps);
+        var projection = !typeMap.DestinationType.IsAbstract
+            ? CreateProjectionCore(request, instanceParameter, typeMap, letPropertyMaps)
+            : Default(typeMap.DestinationType);
+
         foreach(var derivedMap in polymorphicMaps)
         {
             var sourceType = derivedMap.SourceType;

src/UnitTests/Projection/ProjectAbstractWithInheritance.cs

@@ -0,0 +1,72 @@
+namespace AutoMapper.UnitTests.Projection;
+
+public class ProjectAbstractWithInheritance : AutoMapperSpecBase
+{
+    class StepGroup
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+        public ICollection<Step> Steps { get; set; } = new HashSet<Step>();
+    }
+    abstract class Step
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+    }
+    class CheckingStep : Step { }
+    class InstructionStep : Step { }
+
+    class StepGroupModel
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+        public ICollection<StepModel> Steps { get; set; } = new HashSet<StepModel>();
+    }
+    abstract class StepModel
+    {
+        public int Id { get; set; }
+        public string Name { get; set; }
+    }
+    class CheckingStepModel : StepModel { }
+    class InstructionStepModel : StepModel { }
+
+    protected override MapperConfiguration CreateConfiguration()
+    {
+        return new MapperConfiguration(cfg =>
+        {
+            cfg.CreateMap<StepGroup, StepGroupModel>();
+            cfg.CreateMap<Step, StepModel>();
+            cfg.CreateMap<CheckingStep, CheckingStepModel>()
+                .IncludeBase<Step, StepModel>();
+            cfg.CreateMap<InstructionStep, InstructionStepModel>()
+                .IncludeBase<Step, StepModel>();
+        });
+    }
+
+    [Fact]
+    public void ProjectCollectionWithElementInheritingAbstractClassWithoutException()
+    {
+        var stepGroup = new StepGroup
+        {
+            Id = 1,
+            Name = "StepGroup",
+            Steps = new List<Step>
+            {
+                new InstructionStep
+                {
+                    Id = 1,
+                    Name = "InstructionStep"
+                },
+                new CheckingStep
+                {
+                    Id = 2,
+                    Name = "CheckingStep"
+                }
+            }
+        };
+
+        var query = new[] { stepGroup }.AsQueryable();
+
+        Should.NotThrow(() => query.ProjectTo<StepGroupModel>(Configuration).SingleOrDefault());
+    }
+}