Runtime: Allow taking out of inline opaque existentials

Only values stored in the outline boxed existential representation can be
shared.

Author: aschwaighofer

stdlib/public/runtime/Casting.cpp

@@ -1593,7 +1593,10 @@ static void unwrapExistential(OpaqueValue *src,
       auto opaqueContainer = reinterpret_cast<OpaqueExistentialContainer*>(src);
       srcCapturedType = opaqueContainer->Type;
       srcValue = srcType->projectValue(src);
-      canTake = false;
+      // Can't take out of possibly shared existential boxes.
+      canTake = (src == srcValue);
+      assert(canTake == srcCapturedType->getValueWitnesses()->isValueInline() &&
+             "Only inline storage is take-able");
 #else
       auto opaqueContainer = reinterpret_cast<OpaqueExistentialContainer*>(src);
       srcCapturedType = opaqueContainer->Type;
@@ -1653,9 +1656,7 @@ static bool _dynamicCastFromExistential(OpaqueValue *dest,
   } else {
 #ifdef SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS
     assert(!isOutOfLine &&
-           srcType->getRepresentation() !=
-               ExistentialTypeRepresentation::Opaque &&
-           "Should only see inline represenations and no opaque existential");
+           "Should only see inline represenations of existentials");
 #else
     // swift_dynamicCast took or destroyed the value as per the original request
     // We may still have an opaque existential container to deallocate.

stdlib/public/runtime/Metadata.cpp

@@ -2228,8 +2228,13 @@ ExistentialTypeMetadata::mayTakeValue(const OpaqueValue *container) const {
   // Opaque existential containers uniquely own their contained value.
   case ExistentialTypeRepresentation::Opaque:
 #ifdef SWIFT_RUNTIME_ENABLE_COW_EXISTENTIALS
+  {
     // We can't take from a shared existential box without checking uniqueness.
-    return false;
+    auto *opaque =
+        reinterpret_cast<const OpaqueExistentialContainer *>(container);
+    auto *vwt = opaque->Type->getValueWitnesses();
+    return vwt->isValueInline();
+  }
 #else
     return true;
 #endif