[mono][metadata] Fix lookup of matching interface method with covariant returns

src/mono/mono/metadata/class-setup-vtable.c

@@ -1918,6 +1918,33 @@ mono_class_setup_vtable_general (MonoClass *klass, MonoMethod **overrides, int o
 					}
 				}
 
+				if ((vtable [im_slot] == NULL) && klass->parent != NULL) {
+					// For covariant returns we might need to lookup matching virtual methods in parent types
+					// that were overriden with a method that doesn't exactly match interface method signature.
+					gboolean found = FALSE;
+					for (MonoClass *parent_klass = klass->parent; parent_klass != NULL && !found; parent_klass = parent_klass->parent) {
+						gpointer iter = NULL;
+						while ((cm = mono_class_get_virtual_methods (parent_klass, &iter))) {
+							TRACE_INTERFACE_VTABLE ((cm != NULL) && printf ("    For slot %d ('%s'.'%s':'%s'), trying (ancestor) method '%s'.'%s':'%s'... ", im_slot, ic->name_space, ic->name, im->name, cm->klass->name_space, cm->klass->name, cm->name));
+							if ((cm != NULL) && check_interface_method_override (klass, im, cm, MONO_ITF_OVERRIDE_SLOT_EMPTY)) {
+								TRACE_INTERFACE_VTABLE (printf ("[everything ok]: ASSIGNING\n"));
+								found = TRUE;
+								if (vtable [cm->slot]) {
+									// We match the current method was overriding it. If this method will
+									// get overriden again, the interface slot will also be updated
+									vtable [im_slot] = vtable [cm->slot];
+								} else {
+									// We add abstract method in the vtable. This method will be overriden
+									// with the actual implementation once we resolve the abstract method later.
+									// FIXME If klass is abstract, we can end up with abstract method in the vtable. Is this a problem ?
+									vtable [im_slot] = cm;
+								}
+								break;
+							}
+						}
+					}
+				}
+
 				if (vtable [im_slot] == NULL) {
 					if (!(im->flags & METHOD_ATTRIBUTE_ABSTRACT)) {
 						TRACE_INTERFACE_VTABLE (printf ("    Using default iface method %s.\n", mono_method_full_name (im, 1)));

src/tests/Regressions/coreclr/GitHub_78635/test78635.cs

@@ -0,0 +1,118 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+public abstract class BaseProp
+{
+}
+
+public class DerivedProp : BaseProp
+{
+}
+
+public interface ICov
+{
+    BaseProp GetMyProp ();
+}
+
+// Abstract
+public abstract class BaseAbstract
+{
+    public abstract BaseProp GetMyProp ();
+}
+
+// TEST1
+public class DerivedAbstract1 : BaseAbstract, ICov
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED"); return new DerivedProp (); }
+}
+
+// TEST2
+public class DerivedAbstract21 : BaseAbstract
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED21"); return new DerivedProp (); }
+}
+
+public class DerivedAbstract22 : DerivedAbstract21, ICov
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED22"); return new DerivedProp (); }
+}
+
+// TEST3
+public abstract class DerivedAbstract31 : BaseAbstract, ICov
+{
+}
+
+public class DerivedAbstract32 : DerivedAbstract31
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED32"); return new DerivedProp (); }
+}
+
+// Virtual
+public abstract class BaseVirtual
+{
+    public virtual BaseProp GetMyProp () { Console.WriteLine ("BASE"); return null; }
+}
+
+// TEST1
+public class DerivedVirtual1 : BaseVirtual, ICov
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED"); return new DerivedProp (); }
+}
+
+// TEST2
+public class DerivedVirtual21 : BaseVirtual
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED21"); return new DerivedProp (); }
+}
+
+public class DerivedVirtual22 : DerivedVirtual21, ICov
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED22"); return new DerivedProp (); }
+}
+
+// TEST3
+public abstract class DerivedVirtual31 : BaseVirtual, ICov
+{
+}
+
+public class DerivedVirtual32 : DerivedVirtual31
+{
+    public override DerivedProp GetMyProp () { Console.WriteLine ("DERIVED32"); return new DerivedProp (); }
+}
+
+
+public class Program {
+
+    public static void TestInstance (ICov derived)
+    {
+        BaseProp prop = derived.GetMyProp ();
+        if (prop == null)
+            throw new Exception ("Invalid return");
+    }
+
+    public static void Test ()
+    {
+        TestInstance (new DerivedAbstract1 ());
+        TestInstance (new DerivedAbstract22 ());
+        TestInstance (new DerivedAbstract32 ());
+        TestInstance (new DerivedVirtual1 ());
+        TestInstance (new DerivedVirtual22 ());
+        TestInstance (new DerivedVirtual32 ());
+    }
+
+    public static int Main (string[] args)
+    {
+        try {
+            Test ();
+        } catch (Exception e) {
+            Console.WriteLine ("NOK");
+            Console.WriteLine (e);
+            return 1;
+        }
+        Console.WriteLine ("OK");
+        return 100;
+    }
+}
+

src/tests/Regressions/coreclr/GitHub_78635/test78635.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="test78635.cs" />
+  </ItemGroup>
+</Project>