Merge pull request swiftlang#37976 from mikeash/fix-protocol-conformance-KVO-5.5

[5.5][Runtime] Fix crash in protocol conformance checks on KVO artificial subclasses.

Author: mikeash

stdlib/public/runtime/ProtocolConformance.cpp

@@ -169,6 +169,14 @@ static MetadataResponse getSuperclassForMaybeIncompleteMetadata(
   if (!classMetadata)
     return {_swift_class_getSuperclass(metadata), MetadataState::Complete};
 
+#if SWIFT_OBJC_INTEROP
+    // Artificial subclasses are not valid type metadata and
+    // tryGetCompleteMetadataNonblocking will crash on them. However, they're
+    // always fully set up, so we can just skip it and fetch the Subclass field.
+    if (classMetadata->isTypeMetadata() && classMetadata->isArtificialSubclass())
+      return {classMetadata->Superclass, MetadataState::Complete};
+#endif
+
   MetadataState metadataState;
   if (knownMetadataState)
     metadataState = *knownMetadataState;

test/Casting/Casts.swift

@@ -954,4 +954,14 @@ CastsTests.test("Recursive AnyHashable") {
   expectEqual(s.x, p)
 }
 
+#if _runtime(_ObjC)
+CastsTests.test("Artificial subclass protocol conformance") {
+  class SwiftClass: NSObject {}
+  let subclass: AnyClass = objc_allocateClassPair(SwiftClass.self,
+                                                  "ArtificialSwiftSubclass", 0)!
+  objc_registerClassPair(subclass)
+  expectFalse(subclass is P.Type)
+}
+#endif
+
 runAllTests()