Allow reabstraction of default interface methods (#23313)

Allow the runtime to load types with incomplete interface implementations. With this change, we allow (in pseudo-C#):

```csharp
interface IFoo { void Frob() { } } 
interface IBar : IFoo { abstract void IFoo.Frob() }
class Fooer : IBar { }
```

Calling IFoo.Frob on an instance of `Fooer` will result in new exception being thrown because the default implementation of `IFoo.Frob` was re-abstracted by `IBar`.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/vm/methodtablebuilder.cpp b/src/vm/methodtablebuilder.cpp
index b9460264c3..286cd74c0a 100644
--- a/src/vm/methodtablebuilder.cpp
+++ b/src/vm/methodtablebuilder.cpp
@@ -10772,6 +10772,18 @@ BOOL MethodTableBuilder::HasDefaultInterfaceImplementation(bmtRTType *pDeclType,
     if (!pDeclMD->IsAbstract())
         return TRUE;
 
+    // If the method is an abstract MethodImpl, this is a reabstraction:
+    //
+    // interface IFoo { void Frob() { } }
+    // interface IBar : IFoo { abstract void IFoo.Frob() }
+    //
+    // We don't require these to have an implementation because they're final anyway.
+    if (pDeclMD->IsMethodImpl())
+    {
+        assert(pDeclMD->IsFinal());
+        return TRUE;
+    }
+
     int targetSlot = pDeclMD->GetSlot();
 
     // Iterate over all the interfaces this type implements