diff --git a/Sema/Sema.cpp b/Sema/Sema.cpp
index f34aa63..9487ed0 100644
--- a/Sema/Sema.cpp
+++ b/Sema/Sema.cpp
@@ -23,17 +23,25 @@
   TUScope = S;
 }
 
-QualType Sema::GetObjcIdType() {
+/// GetObjcIdType - The following method assumes that "id" is imported
+/// via <objc/objc.h>. This is the way GCC worked for almost 20 years.
+/// In GCC 4.0, "id" is now a built-in type. Unfortunately, typedefs *cannot* be
+/// redefined (even if they are identical). To allow a built-in types to coexist
+/// with <objc/objc.h>, GCC has a special hack on decls (DECL_IN_SYSTEM_HEADER).
+/// For now, we will *not* install id as a built-in. FIXME: reconsider this.
+QualType Sema::GetObjcIdType(SourceLocation Loc) {
   assert(TUScope && "GetObjcIdType(): Top-level scope is null");
-  if (ObjcIdType.isNull()) {
+  if (!ObjcIdTypedef) {
     IdentifierInfo *IdIdent = &Context.Idents.get("id");
     ScopedDecl *IdDecl = LookupScopedDecl(IdIdent, Decl::IDNS_Ordinary, 
                                           SourceLocation(), TUScope);
-    TypedefDecl *IdTypedef = dyn_cast_or_null<TypedefDecl>(IdDecl);
-    assert(IdTypedef && "GetObjcIdType(): Couldn't find 'id' type");
-    ObjcIdType = Context.getTypedefType(IdTypedef);
+    ObjcIdTypedef = dyn_cast_or_null<TypedefDecl>(IdDecl);
+    if (!ObjcIdTypedef) {
+      Diag(Loc, diag::err_missing_id_definition);
+      return QualType();
+    }
   }
-  return ObjcIdType;
+  return Context.getTypedefType(ObjcIdTypedef);
 }
 
 
@@ -56,7 +64,7 @@
   KnownFunctionIDs[ id_vprintf ] = &IT.get("vprintf");
   
   TUScope = 0;
-  ObjcIdType = QualType();
+  ObjcIdTypedef = 0;
 }
 
 void Sema::DeleteExpr(ExprTy *E) {
diff --git a/Sema/Sema.h b/Sema/Sema.h
index 0ac513e..0eb803e 100644
--- a/Sema/Sema.h
+++ b/Sema/Sema.h
@@ -19,7 +19,6 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallPtrSet.h"
-#include "clang/AST/Type.h"
 #include <vector>
 #include <string>
 
@@ -119,8 +118,8 @@
   /// For example, user-defined classes, built-in "id" type, etc.
   Scope *TUScope;
   
-  /// ObjcIdType - built-in type for "id".
-  QualType ObjcIdType;
+  /// ObjcIdTypedef - built-in typedef for "id".
+  TypedefDecl *ObjcIdTypedef;
 public:
   Sema(Preprocessor &pp, ASTContext &ctxt, std::vector<Decl*> &prevInGroup);
   
@@ -257,7 +256,7 @@
                                   const ObjcMethodDecl *PrevMethod); 
 
   /// GetObjcIdType - Getter for the build-in "id" type.
-  QualType GetObjcIdType();
+  QualType GetObjcIdType(SourceLocation Loc = SourceLocation());
   
   //===--------------------------------------------------------------------===//
   // Statement Parsing Callbacks: SemaStmt.cpp.
diff --git a/Sema/SemaDecl.cpp b/Sema/SemaDecl.cpp
index eb1b158..1d15d7b 100644
--- a/Sema/SemaDecl.cpp
+++ b/Sema/SemaDecl.cpp
@@ -1779,7 +1779,7 @@
     if (ArgTypes[i])
       argType = QualType::getFromOpaquePtr(ArgTypes[i]);
     else
-      argType = GetObjcIdType();
+      argType = GetObjcIdType(MethodLoc);
     ParmVarDecl* Param = new ParmVarDecl(SourceLocation(/*FIXME*/), ArgNames[i], 
                                          argType, VarDecl::None, 0);
     Params.push_back(Param);
@@ -1789,7 +1789,7 @@
   if (ReturnType)
     resultDeclType = QualType::getFromOpaquePtr(ReturnType);
   else // get the type for "id".
-    resultDeclType = GetObjcIdType();
+    resultDeclType = GetObjcIdType(MethodLoc);
 
   ObjcMethodDecl* ObjcMethod =  new ObjcMethodDecl(MethodLoc, Sel,
                                       resultDeclType, 0, -1, AttrList, 
diff --git a/clang.xcodeproj/project.pbxproj b/clang.xcodeproj/project.pbxproj
index c7f4908..b5cf88f 100644
--- a/clang.xcodeproj/project.pbxproj
+++ b/clang.xcodeproj/project.pbxproj
@@ -739,7 +739,6 @@
 		08FB7793FE84155DC02AAC07 /* Project object */ = {
 			isa = PBXProject;
 			buildConfigurationList = 1DEB923508733DC60010E9CD /* Build configuration list for PBXProject "clang" */;
-			compatibilityVersion = "Xcode 2.4";
 			hasScannedForEncodings = 1;
 			mainGroup = 08FB7794FE84155DC02AAC07 /* clang */;
 			projectDirPath = "";
diff --git a/include/clang/Basic/DiagnosticKinds.def b/include/clang/Basic/DiagnosticKinds.def
index b71a6a6..adc2425 100644
--- a/include/clang/Basic/DiagnosticKinds.def
+++ b/include/clang/Basic/DiagnosticKinds.def
@@ -444,6 +444,8 @@
      "previous declaration is here")
 DIAG(err_undeclared_protocol, ERROR,
      "cannot find protocol declaration for '%0'")
+DIAG(err_missing_id_definition, ERROR,
+     "cannot find definition of 'id'")
 
 
 //===----------------------------------------------------------------------===//
diff --git a/test/Sema/id_not_builtin.m b/test/Sema/id_not_builtin.m
new file mode 100644
index 0000000..81988fa
--- /dev/null
+++ b/test/Sema/id_not_builtin.m
@@ -0,0 +1,9 @@
+// RUN: clang %s -parse-ast -verify
+
+id obj; // expected-error{{expected '=', ',', ';', 'asm', or '__attribute__' after declarator}}
+
+@interface Foo
+
+- defaultToId; // expected-error{{cannot find definition of 'id'}}
+
+@end