Return a stack block unchanged from objc_retain()

Test/CMakeLists.txt

@@ -33,6 +33,7 @@ set(TESTS
 	ProtocolCreation.m
 	ResurrectInDealloc_arc.m
 	RuntimeTest.m
+	StackBlockRetain_arc.m
 	SuperMethodMissing.m
 	WeakBlock_arc.m
 	WeakRefLoad.m

Test/StackBlockRetain_arc.m

@@ -0,0 +1,37 @@
+#include "Test.h"
+
+// A block that captures a stack block copies it when it is copied itself,
+// and releases that copy when it goes away. What the captured block
+// captured must go with it.
+
+static int deallocs;
+
+@interface Captured : Test
+@end
+@implementation Captured
+- (void)dealloc
+{
+	deallocs++;
+}
+@end
+
+static id kept;
+
+static void keep(void (^block)(void))
+{
+	kept = ^{ block(); };
+}
+
+int main(void)
+{
+	@autoreleasepool
+	{
+		Captured *captured = [Captured new];
+		keep(^{ (void)captured; });
+		captured = nil;
+		assert(deallocs == 0);
+		kept = nil;
+	}
+	assert(deallocs == 1);
+	return 0;
+}

arc.mm

@@ -496,6 +496,10 @@ static inline id retain(id obj, BOOL isWeak)
 	Class cls = obj->isa;
 	if (UNLIKELY(objc_test_class_flag(cls, objc_class_flag_is_block)))
 	{
+		if (cls == static_cast<void*>(&_NSConcreteStackBlock))
+		{
+			return obj;
+		}
 		return Block_copy(obj);
 	}
 	if (objc_test_class_flag(cls, objc_class_flag_fast_arc))