Index: lib/AST/DeclObjC.cpp
===================================================================
--- lib/AST/DeclObjC.cpp
+++ lib/AST/DeclObjC.cpp
@@ -1741,8 +1741,10 @@
 
   } else if (ObjCCategoryImplDecl *ImplD =
              dyn_cast_or_null<ObjCCategoryImplDecl>(this)) {
-    if (ObjCCategoryDecl *CD = IFace->FindCategoryDeclaration(getIdentifier()))
+    if (ObjCCategoryDecl *CD =
+        IFace->FindCategoryDeclaration(ImplD->getIdentifier())) {
       Ctx.setObjCImplementation(CD, ImplD);
+    }
   }
 
   ClassInterface = IFace;
Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -981,17 +981,20 @@
 
 void ASTDeclReader::VisitObjCImplDecl(ObjCImplDecl *D) {
   VisitObjCContainerDecl(D);
-  D->setClassInterface(ReadDeclAs<ObjCInterfaceDecl>(Record, Idx));
+  // For ObjCCategoryImplDecl, setClassInterface must not be called until the
+  // category identifier has been deserialized and set.  See PR23175.
 }
 
 void ASTDeclReader::VisitObjCCategoryImplDecl(ObjCCategoryImplDecl *D) {
   VisitObjCImplDecl(D);
   D->setIdentifier(Reader.GetIdentifierInfo(F, Record, Idx));
   D->CategoryNameLoc = ReadSourceLocation(Record, Idx);
+  D->setClassInterface(ReadDeclAs<ObjCInterfaceDecl>(Record, Idx));
 }
 
 void ASTDeclReader::VisitObjCImplementationDecl(ObjCImplementationDecl *D) {
   VisitObjCImplDecl(D);
+  D->setClassInterface(ReadDeclAs<ObjCInterfaceDecl>(Record, Idx));
   D->setSuperClass(ReadDeclAs<ObjCInterfaceDecl>(Record, Idx));
   D->SuperLoc = ReadSourceLocation(Record, Idx);
   D->setIvarLBraceLoc(ReadSourceLocation(Record, Idx));
Index: lib/Serialization/ASTWriterDecl.cpp
===================================================================
--- lib/Serialization/ASTWriterDecl.cpp
+++ lib/Serialization/ASTWriterDecl.cpp
@@ -695,7 +695,6 @@
 
 void ASTDeclWriter::VisitObjCImplDecl(ObjCImplDecl *D) {
   VisitObjCContainerDecl(D);
-  Writer.AddDeclRef(D->getClassInterface(), Record);
   // Abstract class (no need to define a stable serialization::DECL code).
 }
 
@@ -703,11 +702,13 @@
   VisitObjCImplDecl(D);
   Writer.AddIdentifierRef(D->getIdentifier(), Record);
   Writer.AddSourceLocation(D->getCategoryNameLoc(), Record);
+  Writer.AddDeclRef(D->getClassInterface(), Record);
   Code = serialization::DECL_OBJC_CATEGORY_IMPL;
 }
 
 void ASTDeclWriter::VisitObjCImplementationDecl(ObjCImplementationDecl *D) {
   VisitObjCImplDecl(D);
+  Writer.AddDeclRef(D->getClassInterface(), Record);
   Writer.AddDeclRef(D->getSuperClass(), Record);
   Writer.AddSourceLocation(D->getSuperClassLoc(), Record);
   Writer.AddSourceLocation(D->getIvarLBraceLoc(), Record);