Fix dtable rebase and add a test for a missing case

The dtable rebasing rework in b81df02b broke the inheritance of
superclass methods from a superclass that had already been initialized.

Test/setSuperclass.m

@@ -32,6 +32,12 @@ __attribute__((objc_root_class))
 @interface InitializesOneClassWhileBeingInitialized: NotInitializedSuperclass2 @end
 @interface Subclass3: DefaultSuperclass @end
 
+// test: transitioning an initialized class to an initialized superclass
+// Test4Subclass only inherits access to "onlyExistsOnFinalSuperclass" from
+// its new superclass's superclass (Test4FinalSuperclass)
+@interface Test4Subclass: Root @end
+@interface Test4FinalSuperclass: DefaultSuperclass @end
+
 @implementation Root
 + (Class)class { return self; }
 + (BOOL)respondsToSelector:(SEL)selector {
@@ -141,6 +147,13 @@ static BOOL _otherInitializedClassInitialized = NO;
 @implementation Subclass3
 @end
 
+@implementation Test4Subclass
+@end
+@implementation Test4FinalSuperclass
++ (int)onlyExistsOnFinalSuperclassMeta { return 501; }
+- (int)onlyExistsOnFinalSuperclass { return 500; }
+@end
+
 static int failures = 0;
 
 #define expect(x) do \
@@ -293,5 +306,28 @@ int main(int argc, char **argv) {
 		expect(13 == [Subclass3 overriddenMeta]);
 	}
 
+	/* Transitioning an initialized class to an initialized superclass. */
+	{
+		Class test4subclass = objc_getClass("Test4Subclass");
+		Class newSuperclass = objc_getClass("Test4FinalSuperclass");
+
+		// Make sure every class in the hierarchy is initialized.
+		[Test4Subclass class];
+		[Test4FinalSuperclass class];
+
+		expect(![test4subclass respondsToSelector:@selector(onlyExistsOnFinalSuperclassMeta)]);
+		expect(![test4subclass instancesRespondToSelector:@selector(onlyExistsOnFinalSuperclass)]);
+
+		class_setSuperclass(test4subclass, newSuperclass);
+
+		Test4Subclass *test4instance = class_createInstance(test4subclass, 0);
+
+		expect([test4subclass respondsToSelector:@selector(onlyExistsOnFinalSuperclassMeta)]);
+		expect([test4subclass instancesRespondToSelector:@selector(onlyExistsOnFinalSuperclass)]);
+
+		expect(501 == [(id)test4subclass onlyExistsOnFinalSuperclassMeta]);
+		expect(500 == [(id)test4instance onlyExistsOnFinalSuperclass]);
+	}
+
 	return failures;
 }

dtable.c

@@ -419,21 +419,24 @@ static void rebaseDtableRecursive(Class cls, Class newSuper)
 	dtable_t dtable = dtable_for_class(cls);
 	uint32_t idx = 0;
 	struct objc_method *method;
-	// Install all methods in the dtable with the correct ones.
-	while ((method = SparseArrayNext(temporaryDtable, &idx)))
+	// Install all methods from the parent that aren't overridden here.
+	while ((method = SparseArrayNext(parentDtable, &idx)))
 	{
-		SparseArrayInsert(dtable, idx, method);
+		if (SparseArrayLookup(temporaryDtable, idx) == NULL)
+		{
+			SparseArrayInsert(dtable, idx, method);
+			SparseArrayInsert(temporaryDtable, idx, method);
+		}
 	}
 	idx = 0;
 	// Now look at all of the methods in the dtable.  If they're not ones from
-	// the dtable that we've just created, then they must come from the
-	// superclass, so replace them with whatever the superclass has (which may
-	// be NULL).
+	// the dtable that we've just created, then they must have come from the
+	// original superclass, so remove them by replacing them with NULL.
 	while ((method = SparseArrayNext(dtable, &idx)))
 	{
 		if (SparseArrayLookup(temporaryDtable, idx) == NULL)
 		{
-			SparseArrayInsert(dtable, idx, SparseArrayLookup(parentDtable, idx));
+			SparseArrayInsert(dtable, idx, NULL);
 		}
 	}
 	SparseArrayDestroy(temporaryDtable);