IRGen: Workaround for inadvertent mangling of opened archetypes

lib/IRGen/GenHeap.cpp

@@ -1528,6 +1528,19 @@ public:
       boxedInterfaceType = boxedType.mapTypeOutOfContext();
     }
 
+    {
+      // FIXME: This seems wrong. We used to just mangle opened archetypes as
+      // their interface type. Let's make that explicit now.
+      auto astType = boxedInterfaceType.getASTType();
+      astType = astType.transformRec([](Type t) -> Optional<Type> {
+        if (auto *openedExistential = t->getAs<OpenedArchetypeType>())
+          return openedExistential->getInterfaceType();
+        return None;
+      })->getCanonicalType();
+      boxedInterfaceType = SILType::getPrimitiveType(
+          astType, boxedInterfaceType.getCategory());
+    }
+
     auto boxDescriptor = IGF.IGM.getAddrOfBoxDescriptor(
         boxedInterfaceType,
         env ? env->getGenericSignature().getCanonicalSignature()

test/Reflection/box_descriptor_with_opened_archetype.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -emit-ir %s -disable-availability-checking
+
+public protocol P {}
+
+struct S<T>: P {
+  var x: Any
+  init() { fatalError() }
+}
+
+public func mangleArchetype(_ p: any P) -> any P {
+  p.open
+}
+
+extension P {
+  var open: some P {
+    S<Self>()
+  }
+}