diff --git a/Makefile.am b/Makefile.am
index 04b4f4a..023afa4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -535,6 +535,7 @@
   objectivec/Tests/GPBDictionaryTests+UInt64.m                               \
   objectivec/Tests/GPBDictionaryTests.m                                      \
   objectivec/Tests/GPBDictionaryTests.pddm                                   \
+  objectivec/Tests/GPBExtensionRegistryTest.m                                \
   objectivec/Tests/GPBMessageTests+Merge.m                                   \
   objectivec/Tests/GPBMessageTests+Runtime.m                                 \
   objectivec/Tests/GPBMessageTests+Serialization.m                           \
diff --git a/objectivec/GPBExtensionRegistry.m b/objectivec/GPBExtensionRegistry.m
index 65534b6..b056a52 100644
--- a/objectivec/GPBExtensionRegistry.m
+++ b/objectivec/GPBExtensionRegistry.m
@@ -57,14 +57,16 @@
 
 - (instancetype)copyWithZone:(NSZone *)zone {
   GPBExtensionRegistry *result = [[[self class] allocWithZone:zone] init];
-  if (result && mutableClassMap_.count) {
-    [result->mutableClassMap_ addEntriesFromDictionary:mutableClassMap_];
-  }
+  [result addExtensions:self];
   return result;
 }
 
-- (CFMutableDictionaryRef)extensionMapForContainingMessageClass:
-        (Class)containingMessageClass {
+- (void)addExtension:(GPBExtensionDescriptor *)extension {
+  if (extension == nil) {
+    return;
+  }
+
+  Class containingMessageClass = extension.containingMessageClass;
   CFMutableDictionaryRef extensionMap = (CFMutableDictionaryRef)
       [mutableClassMap_ objectForKey:containingMessageClass];
   if (extensionMap == nil) {
@@ -74,18 +76,9 @@
                                              &kCFTypeDictionaryValueCallBacks);
     [mutableClassMap_ setObject:(id)extensionMap
                          forKey:(id<NSCopying>)containingMessageClass];
-  }
-  return extensionMap;
-}
-
-- (void)addExtension:(GPBExtensionDescriptor *)extension {
-  if (extension == nil) {
-    return;
+    CFRelease(extensionMap);
   }
 
-  Class containingMessageClass = extension.containingMessageClass;
-  CFMutableDictionaryRef extensionMap =
-      [self extensionMapForContainingMessageClass:containingMessageClass];
   ssize_t key = extension.fieldNumber;
   CFDictionarySetValue(extensionMap, (const void *)key, extension);
 }
@@ -119,10 +112,16 @@
     Class containingMessageClass = key;
     CFMutableDictionaryRef otherExtensionMap = (CFMutableDictionaryRef)value;
 
-    CFMutableDictionaryRef extensionMap =
-        [self extensionMapForContainingMessageClass:containingMessageClass];
-
-    CFDictionaryApplyFunction(otherExtensionMap, CopyKeyValue, extensionMap);
+    CFMutableDictionaryRef extensionMap = (CFMutableDictionaryRef)
+        [mutableClassMap_ objectForKey:containingMessageClass];
+    if (extensionMap == nil) {
+      extensionMap = CFDictionaryCreateMutableCopy(kCFAllocatorDefault, 0, otherExtensionMap);
+      [mutableClassMap_ setObject:(id)extensionMap
+                           forKey:(id<NSCopying>)containingMessageClass];
+      CFRelease(extensionMap);
+    } else {
+      CFDictionaryApplyFunction(otherExtensionMap, CopyKeyValue, extensionMap);
+    }
   }];
 }
 
diff --git a/objectivec/ProtocolBuffers_OSX.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_OSX.xcodeproj/project.pbxproj
index 919d007..cd7fcc9 100644
--- a/objectivec/ProtocolBuffers_OSX.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_OSX.xcodeproj/project.pbxproj
@@ -52,6 +52,7 @@
 		F4487C751AADF7F500531423 /* GPBMessageTests+Runtime.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C741AADF7F500531423 /* GPBMessageTests+Runtime.m */; };
 		F4487C7F1AAF62CD00531423 /* GPBMessageTests+Serialization.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C7E1AAF62CD00531423 /* GPBMessageTests+Serialization.m */; };
 		F4487C831AAF6AB300531423 /* GPBMessageTests+Merge.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C821AAF6AB300531423 /* GPBMessageTests+Merge.m */; };
+		F4584D821ECCB52A00803AB6 /* GPBExtensionRegistryTest.m in Sources */ = {isa = PBXBuildFile; fileRef = F4584D7E1ECCB38900803AB6 /* GPBExtensionRegistryTest.m */; };
 		F45C69CC16DFD08D0081955B /* GPBExtensionInternals.m in Sources */ = {isa = PBXBuildFile; fileRef = F45C69CB16DFD08D0081955B /* GPBExtensionInternals.m */; };
 		F45E57C71AE6DC6A000B7D99 /* text_format_map_unittest_data.txt in Resources */ = {isa = PBXBuildFile; fileRef = F45E57C61AE6DC6A000B7D99 /* text_format_map_unittest_data.txt */; };
 		F47476E51D21A524007C7B1A /* Duration.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = 8B4248D41A92826400BC1EC6 /* Duration.pbobjc.m */; };
@@ -180,6 +181,7 @@
 		F4487C821AAF6AB300531423 /* GPBMessageTests+Merge.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = "GPBMessageTests+Merge.m"; sourceTree = "<group>"; };
 		F44929001C866B1900C2548A /* GPBCodedOutputStream_PackagePrivate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = GPBCodedOutputStream_PackagePrivate.h; sourceTree = "<group>"; };
 		F451D3F51A8AAE8700B8A22C /* GPBProtocolBuffers_RuntimeSupport.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GPBProtocolBuffers_RuntimeSupport.h; sourceTree = "<group>"; };
+		F4584D7E1ECCB38900803AB6 /* GPBExtensionRegistryTest.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBExtensionRegistryTest.m; sourceTree = "<group>"; };
 		F45C69CB16DFD08D0081955B /* GPBExtensionInternals.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBExtensionInternals.m; sourceTree = "<group>"; };
 		F45E57C61AE6DC6A000B7D99 /* text_format_map_unittest_data.txt */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = text_format_map_unittest_data.txt; sourceTree = "<group>"; };
 		F4AC9E1D1A8BEB3500BD6E83 /* unittest_cycle.proto */ = {isa = PBXFileReference; lastKnownFileType = text; path = unittest_cycle.proto; sourceTree = "<group>"; };
@@ -403,6 +405,7 @@
 				F4353D301AC06F10005A6198 /* GPBDictionaryTests+String.m */,
 				F4353D311AC06F10005A6198 /* GPBDictionaryTests+UInt32.m */,
 				F4353D321AC06F10005A6198 /* GPBDictionaryTests+UInt64.m */,
+				F4584D7E1ECCB38900803AB6 /* GPBExtensionRegistryTest.m */,
 				7461B6A30F94FDF800A0C422 /* GPBMessageTests.m */,
 				F4487C821AAF6AB300531423 /* GPBMessageTests+Merge.m */,
 				F4487C741AADF7F500531423 /* GPBMessageTests+Runtime.m */,
@@ -418,8 +421,8 @@
 				7461B6BA0F94FDF900A0C422 /* GPBUtilitiesTests.m */,
 				8B4248DB1A92933A00BC1EC6 /* GPBWellKnownTypesTest.m */,
 				7461B6BC0F94FDF900A0C422 /* GPBWireFormatTests.m */,
-				F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */,
 				F45E57C61AE6DC6A000B7D99 /* text_format_map_unittest_data.txt */,
+				F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */,
 				8B7E6A7414893DBA00F8884A /* unittest_custom_options.proto */,
 				F4AC9E1D1A8BEB3500BD6E83 /* unittest_cycle.proto */,
 				8B7E6A7514893DBA00F8884A /* unittest_embed_optimize_for.proto */,
@@ -429,8 +432,8 @@
 				8BBD9DB016DD1DC8008E1EC1 /* unittest_lite.proto */,
 				8B7E6A7B14893DBC00F8884A /* unittest_mset.proto */,
 				8B7E6A7C14893DBC00F8884A /* unittest_no_generic_services.proto */,
-				8B09AAF614B663A7007B4184 /* unittest_objc.proto */,
 				F4CF31701B162ED800BD9B06 /* unittest_objc_startup.proto */,
+				8B09AAF614B663A7007B4184 /* unittest_objc.proto */,
 				8B7E6A7D14893DBC00F8884A /* unittest_optimize_for.proto */,
 				F4487C781AADFB3100531423 /* unittest_runtime_proto2.proto */,
 				F4487C791AADFB3200531423 /* unittest_runtime_proto3.proto */,
@@ -669,6 +672,7 @@
 				F4F8D8831D789FD9002CE128 /* GPBUnittestProtos2.m in Sources */,
 				F4353D1D1AB8822D005A6198 /* GPBDescriptorTests.m in Sources */,
 				8B4248BB1A8C256A00BC1EC6 /* GPBSwiftTests.swift in Sources */,
+				F4584D821ECCB52A00803AB6 /* GPBExtensionRegistryTest.m in Sources */,
 				5102DABC1891A073002037B6 /* GPBConcurrencyTests.m in Sources */,
 				F4487C751AADF7F500531423 /* GPBMessageTests+Runtime.m in Sources */,
 				F4353D351AC06F10005A6198 /* GPBDictionaryTests+Int32.m in Sources */,
diff --git a/objectivec/ProtocolBuffers_OSX.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme b/objectivec/ProtocolBuffers_OSX.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
index 2f61813..2883109 100644
--- a/objectivec/ProtocolBuffers_OSX.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
+++ b/objectivec/ProtocolBuffers_OSX.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
@@ -51,6 +51,12 @@
                   Identifier = "DescriptorTests">
                </Test>
                <Test
+                  Identifier = "GPBAutocreatedArrayTests">
+               </Test>
+               <Test
+                  Identifier = "GPBAutocreatedDictionaryTests">
+               </Test>
+               <Test
                   Identifier = "GPBBoolArrayTests">
                </Test>
                <Test
@@ -90,6 +96,9 @@
                   Identifier = "GPBEnumArrayTests">
                </Test>
                <Test
+                  Identifier = "GPBExtensionRegistryTest">
+               </Test>
+               <Test
                   Identifier = "GPBFloatArrayTests">
                </Test>
                <Test
diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
index 64fc45c..2211cb3 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
@@ -60,6 +60,7 @@
 		F4487C771AADF84900531423 /* GPBMessageTests+Runtime.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C761AADF84900531423 /* GPBMessageTests+Runtime.m */; };
 		F4487C811AAF62FC00531423 /* GPBMessageTests+Serialization.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C801AAF62FC00531423 /* GPBMessageTests+Serialization.m */; };
 		F4487C851AAF6AC500531423 /* GPBMessageTests+Merge.m in Sources */ = {isa = PBXBuildFile; fileRef = F4487C841AAF6AC500531423 /* GPBMessageTests+Merge.m */; };
+		F4584D831ECCB53600803AB6 /* GPBExtensionRegistryTest.m in Sources */ = {isa = PBXBuildFile; fileRef = F4584D801ECCB39E00803AB6 /* GPBExtensionRegistryTest.m */; };
 		F45C69CC16DFD08D0081955B /* GPBExtensionInternals.m in Sources */ = {isa = PBXBuildFile; fileRef = F45C69CB16DFD08D0081955B /* GPBExtensionInternals.m */; };
 		F45E57C91AE6DC98000B7D99 /* text_format_map_unittest_data.txt in Resources */ = {isa = PBXBuildFile; fileRef = F45E57C81AE6DC98000B7D99 /* text_format_map_unittest_data.txt */; };
 		F47476E91D21A537007C7B1A /* Duration.pbobjc.m in Sources */ = {isa = PBXBuildFile; fileRef = 8B4248DE1A929C7D00BC1EC6 /* Duration.pbobjc.m */; };
@@ -202,6 +203,7 @@
 		F4487C841AAF6AC500531423 /* GPBMessageTests+Merge.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = "GPBMessageTests+Merge.m"; sourceTree = "<group>"; };
 		F44929021C866B3B00C2548A /* GPBCodedOutputStream_PackagePrivate.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = GPBCodedOutputStream_PackagePrivate.h; sourceTree = "<group>"; };
 		F451D3F61A8AAEA600B8A22C /* GPBProtocolBuffers_RuntimeSupport.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = GPBProtocolBuffers_RuntimeSupport.h; sourceTree = "<group>"; };
+		F4584D801ECCB39E00803AB6 /* GPBExtensionRegistryTest.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = GPBExtensionRegistryTest.m; path = Tests/GPBExtensionRegistryTest.m; sourceTree = SOURCE_ROOT; };
 		F45C69CB16DFD08D0081955B /* GPBExtensionInternals.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = GPBExtensionInternals.m; sourceTree = "<group>"; };
 		F45E57C81AE6DC98000B7D99 /* text_format_map_unittest_data.txt */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = text_format_map_unittest_data.txt; sourceTree = "<group>"; };
 		F4AC9E1C1A8BEB1000BD6E83 /* unittest_cycle.proto */ = {isa = PBXFileReference; lastKnownFileType = text; path = unittest_cycle.proto; sourceTree = "<group>"; };
@@ -441,6 +443,7 @@
 				F4353D3E1AC06F31005A6198 /* GPBDictionaryTests+String.m */,
 				F4353D3F1AC06F31005A6198 /* GPBDictionaryTests+UInt32.m */,
 				F4353D401AC06F31005A6198 /* GPBDictionaryTests+UInt64.m */,
+				F4584D801ECCB39E00803AB6 /* GPBExtensionRegistryTest.m */,
 				7461B6A30F94FDF800A0C422 /* GPBMessageTests.m */,
 				F4487C841AAF6AC500531423 /* GPBMessageTests+Merge.m */,
 				F4487C761AADF84900531423 /* GPBMessageTests+Runtime.m */,
@@ -456,8 +459,8 @@
 				7461B6BA0F94FDF900A0C422 /* GPBUtilitiesTests.m */,
 				8B4248E51A929C9900BC1EC6 /* GPBWellKnownTypesTest.m */,
 				7461B6BC0F94FDF900A0C422 /* GPBWireFormatTests.m */,
-				F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */,
 				F45E57C81AE6DC98000B7D99 /* text_format_map_unittest_data.txt */,
+				F43C88CF191D77FC009E917D /* text_format_unittest_data.txt */,
 				8B7E6A7414893DBA00F8884A /* unittest_custom_options.proto */,
 				F4AC9E1C1A8BEB1000BD6E83 /* unittest_cycle.proto */,
 				8B7E6A7514893DBA00F8884A /* unittest_embed_optimize_for.proto */,
@@ -467,8 +470,8 @@
 				8BBD9DB016DD1DC8008E1EC1 /* unittest_lite.proto */,
 				8B7E6A7B14893DBC00F8884A /* unittest_mset.proto */,
 				8B7E6A7C14893DBC00F8884A /* unittest_no_generic_services.proto */,
-				8B09AAF614B663A7007B4184 /* unittest_objc.proto */,
 				F4CF31711B162EF500BD9B06 /* unittest_objc_startup.proto */,
+				8B09AAF614B663A7007B4184 /* unittest_objc.proto */,
 				8B7E6A7D14893DBC00F8884A /* unittest_optimize_for.proto */,
 				F4487C7A1AADFB5500531423 /* unittest_runtime_proto2.proto */,
 				F4487C7B1AADFB5500531423 /* unittest_runtime_proto3.proto */,
@@ -765,6 +768,7 @@
 				F4F8D8861D78A193002CE128 /* GPBUnittestProtos2.m in Sources */,
 				F4B51B1C1BBC5C7100744318 /* GPBObjectiveCPlusPlusTest.mm in Sources */,
 				8B4248B41A8BD96E00BC1EC6 /* GPBSwiftTests.swift in Sources */,
+				F4584D831ECCB53600803AB6 /* GPBExtensionRegistryTest.m in Sources */,
 				5102DABC1891A073002037B6 /* GPBConcurrencyTests.m in Sources */,
 				F4487C771AADF84900531423 /* GPBMessageTests+Runtime.m in Sources */,
 				F4353D431AC06F31005A6198 /* GPBDictionaryTests+Int32.m in Sources */,
diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme b/objectivec/ProtocolBuffers_iOS.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
index be31c30..1ba3a32 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/xcshareddata/xcschemes/PerformanceTests.xcscheme
@@ -51,6 +51,12 @@
                   Identifier = "DescriptorTests">
                </Test>
                <Test
+                  Identifier = "GPBAutocreatedArrayTests">
+               </Test>
+               <Test
+                  Identifier = "GPBAutocreatedDictionaryTests">
+               </Test>
+               <Test
                   Identifier = "GPBBoolArrayTests">
                </Test>
                <Test
@@ -90,6 +96,9 @@
                   Identifier = "GPBEnumArrayTests">
                </Test>
                <Test
+                  Identifier = "GPBExtensionRegistryTest">
+               </Test>
+               <Test
                   Identifier = "GPBFloatArrayTests">
                </Test>
                <Test
diff --git a/objectivec/Tests/GPBExtensionRegistryTest.m b/objectivec/Tests/GPBExtensionRegistryTest.m
new file mode 100644
index 0000000..b116882
--- /dev/null
+++ b/objectivec/Tests/GPBExtensionRegistryTest.m
@@ -0,0 +1,138 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2017 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBTestUtilities.h"
+
+#import "GPBExtensionRegistry.h"
+#import "google/protobuf/Unittest.pbobjc.h"
+
+@interface GPBExtensionRegistryTest : GPBTestCase
+@end
+
+@implementation GPBExtensionRegistryTest
+
+- (void)testBasics {
+  GPBExtensionRegistry *reg = [[[GPBExtensionRegistry alloc] init] autorelease];
+  XCTAssertNotNil(reg);
+
+  XCTAssertNil([reg extensionForDescriptor:[TestAllExtensions descriptor]
+                               fieldNumber:1]);
+  XCTAssertNil([reg extensionForDescriptor:[TestAllTypes descriptor]
+                               fieldNumber:1]);
+
+  [reg addExtension:[UnittestRoot optionalInt32Extension]];
+  [reg addExtension:[UnittestRoot packedInt64Extension]];
+
+  XCTAssertTrue([reg extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:1] ==
+                [UnittestRoot optionalInt32Extension]); // ptr equality
+  XCTAssertNil([reg extensionForDescriptor:[TestAllTypes descriptor]
+                               fieldNumber:1]);
+  XCTAssertTrue([reg extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:91] ==
+                [UnittestRoot packedInt64Extension]); // ptr equality
+}
+
+- (void)testCopy {
+  GPBExtensionRegistry *reg1 = [[[GPBExtensionRegistry alloc] init] autorelease];
+  [reg1 addExtension:[UnittestRoot optionalInt32Extension]];
+
+  GPBExtensionRegistry *reg2 = [[reg1 copy] autorelease];
+  XCTAssertNotNil(reg2);
+
+  XCTAssertTrue([reg1 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:1] ==
+                [UnittestRoot optionalInt32Extension]); // ptr equality
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:1] ==
+                [UnittestRoot optionalInt32Extension]); // ptr equality
+
+  // Message class that had registered extension(s) at the -copy time.
+
+  [reg1 addExtension:[UnittestRoot optionalBoolExtension]];
+  [reg2 addExtension:[UnittestRoot optionalStringExtension]];
+
+  XCTAssertTrue([reg1 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:13] ==
+                [UnittestRoot optionalBoolExtension]); // ptr equality
+  XCTAssertNil([reg1 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:14]);
+  XCTAssertNil([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:13]);
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:14] ==
+                [UnittestRoot optionalStringExtension]); // ptr equality
+
+  // Message class that did not have any registered extensions at the -copy time.
+
+  [reg1 addExtension:[UnittestRoot packedInt64Extension]];
+  [reg2 addExtension:[UnittestRoot packedSint32Extension]];
+
+  XCTAssertTrue([reg1 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:91] ==
+                [UnittestRoot packedInt64Extension]); // ptr equality
+  XCTAssertNil([reg1 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:94]);
+  XCTAssertNil([reg2 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:91]);
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:94] ==
+                [UnittestRoot packedSint32Extension]); // ptr equality
+
+}
+
+- (void)testAddExtensions {
+  GPBExtensionRegistry *reg1 = [[[GPBExtensionRegistry alloc] init] autorelease];
+  [reg1 addExtension:[UnittestRoot optionalInt32Extension]];
+
+  GPBExtensionRegistry *reg2 = [[[GPBExtensionRegistry alloc] init] autorelease];
+
+  XCTAssertNil([reg2 extensionForDescriptor:[TestAllExtensions descriptor]
+                                fieldNumber:1]);
+
+  [reg2 addExtensions:reg1];
+
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:1] ==
+                [UnittestRoot optionalInt32Extension]); // ptr equality
+
+  // Confirm adding to the first doesn't add to the second.
+
+  [reg1 addExtension:[UnittestRoot optionalBoolExtension]];
+  [reg1 addExtension:[UnittestRoot packedInt64Extension]];
+
+  XCTAssertTrue([reg1 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:13] ==
+                [UnittestRoot optionalBoolExtension]); // ptr equality
+  XCTAssertTrue([reg1 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:91] ==
+                [UnittestRoot packedInt64Extension]); // ptr equality
+  XCTAssertNil([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:13]);
+  XCTAssertNil([reg2 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:91]);
+
+  // Confirm adding to the second doesn't add to the first.
+
+  [reg2 addExtension:[UnittestRoot optionalStringExtension]];
+  [reg2 addExtension:[UnittestRoot packedSint32Extension]];
+
+  XCTAssertNil([reg1 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:14]);
+  XCTAssertNil([reg1 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:94]);
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestAllExtensions descriptor] fieldNumber:14] ==
+                [UnittestRoot optionalStringExtension]); // ptr equality
+  XCTAssertTrue([reg2 extensionForDescriptor:[TestPackedExtensions descriptor] fieldNumber:94] ==
+                [UnittestRoot packedSint32Extension]); // ptr equality
+}
+
+@end