diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 7a5d22c00f8f..046c699a224f 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1205,6 +1205,12 @@ void ASTDeclReader::VisitObjCCategoryDecl(ObjCCategoryDecl *CD) {
     ProtoLocs.push_back(ReadSourceLocation());
   CD->setProtocolList(ProtoRefs.data(), NumProtoRefs, ProtoLocs.data(),
                       Reader.getContext());
+
+  // Protocols in the class extension belong to the class.
+  if (NumProtoRefs > 0 && CD->ClassInterface && CD->IsClassExtension())
+    CD->ClassInterface->mergeClassExtensionProtocolList(
+        (ObjCProtocolDecl *const *)ProtoRefs.data(), NumProtoRefs,
+        Reader.getContext());
 }
 
 void ASTDeclReader::VisitObjCCompatibleAliasDecl(ObjCCompatibleAliasDecl *CAD) {
diff --git a/clang/test/Modules/Inputs/class-extension/a-private.h b/clang/test/Modules/Inputs/class-extension/a-private.h
new file mode 100644
index 000000000000..e35402ac18ad
--- /dev/null
+++ b/clang/test/Modules/Inputs/class-extension/a-private.h
@@ -0,0 +1,5 @@
+#import "a.h"
+#import "a-proto.h"
+
+@interface A () <AProto>
+@end
diff --git a/clang/test/Modules/Inputs/class-extension/a-proto.h b/clang/test/Modules/Inputs/class-extension/a-proto.h
new file mode 100644
index 000000000000..32043ec2498f
--- /dev/null
+++ b/clang/test/Modules/Inputs/class-extension/a-proto.h
@@ -0,0 +1,7 @@
+@protocol NSObject
+@end
+
+@protocol AProto <NSObject>
+@property (nonatomic, readwrite, assign) int p0;
+@property (nonatomic, readwrite, assign) int p1;
+@end
diff --git a/clang/test/Modules/Inputs/class-extension/a.h b/clang/test/Modules/Inputs/class-extension/a.h
new file mode 100644
index 000000000000..28c409c33774
--- /dev/null
+++ b/clang/test/Modules/Inputs/class-extension/a.h
@@ -0,0 +1,5 @@
+@interface NSObject
+@end
+
+@interface A : NSObject
+@end
diff --git a/clang/test/Modules/Inputs/class-extension/module.modulemap b/clang/test/Modules/Inputs/class-extension/module.modulemap
new file mode 100644
index 000000000000..5c30bc57e46c
--- /dev/null
+++ b/clang/test/Modules/Inputs/class-extension/module.modulemap
@@ -0,0 +1,11 @@
+
+module A {
+  header "a.h"
+  header "a-proto.h"
+  export *
+}
+
+module AP {
+  header "a-private.h"
+  export *
+}
diff --git a/clang/test/Modules/class-extension-protocol.m b/clang/test/Modules/class-extension-protocol.m
new file mode 100644
index 000000000000..752e4e129d29
--- /dev/null
+++ b/clang/test/Modules/class-extension-protocol.m
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t.cache
+// RUN: %clang_cc1 %s -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t.cache -I%S/Inputs/class-extension -verify
+// expected-no-diagnostics
+
+#import "a-private.h"
+
+int foo(A *X) {
+  return X.p0 + X.p1;
+}