readonly members can be in fact mapped

Author: lbargaoanu

src/AutoMapper/Configuration/MemberConfigurationExpression.cs

@@ -130,7 +130,7 @@ public void Configure(TypeMap typeMap)
         var destMember = DestinationMember;
         if(destMember.DeclaringType.ContainsGenericParameters)
         {
-            destMember = typeMap.DestinationSetters.Single(m => m.Name == destMember.Name);
+            destMember = Array.Find(typeMap.DestinationTypeDetails.ReadAccessors, m => m.MetadataToken == destMember.MetadataToken);
         }
         var propertyMap = typeMap.FindOrCreatePropertyMapFor(destMember, typeof(TMember) == typeof(object) ? destMember.GetMemberType() : typeof(TMember));
         Apply(propertyMap);

src/AutoMapper/TypeMap.cs

@@ -65,7 +65,7 @@ internal bool CanConstructorMap() => Profile.ConstructorMappingEnabled && !Desti
     public TypePair Types;
     public ConstructorMap ConstructorMap { get; set; }
     public TypeDetails SourceTypeDetails { get; private set; }
-    private TypeDetails DestinationTypeDetails { get; set; }
+    public TypeDetails DestinationTypeDetails { get; private set; }
     public Type SourceType => Types.SourceType;
     public Type DestinationType => Types.DestinationType;
     public ProfileMap Profile { get; }

src/UnitTests/OpenGenerics.cs

@@ -1,4 +1,30 @@
 namespace AutoMapper.UnitTests;
+public class ReadonlyPropertiesGenerics : AutoMapperSpecBase
+{
+    class Source
+    {
+        public InnerSource Inner;
+    }
+    class InnerSource
+    {
+        public int Value;
+    }
+    class Destination<T>
+    {
+        public readonly InnerDestination Inner = new();
+    }
+    class InnerDestination
+    {
+        public int Value;
+    }
+    protected override MapperConfiguration CreateConfiguration() => new(c =>
+    {
+        c.CreateMap(typeof(Source), typeof(Destination<>)).ForMember("Inner", o => o.MapFrom("Inner"));
+        c.CreateMap<InnerSource, InnerDestination>();
+    });
+    [Fact]
+    public void Should_work() => Map<Destination<int>>(new Source { Inner = new() { Value = 42 } }).Inner.Value.ShouldBe(42);
+}
 public class ConstructorValidationGenerics : NonValidatingSpecBase
 {
     record Source<T>(T Value);