diff --git a/include/clang/AST/ExprCXX.h b/include/clang/AST/ExprCXX.h
index 88e667a..0eaae29 100644
--- a/include/clang/AST/ExprCXX.h
+++ b/include/clang/AST/ExprCXX.h
@@ -386,15 +386,30 @@
 /// parameter's default argument, when the call did not explicitly
 /// supply arguments for all of the parameters.
 class CXXDefaultArgExpr : public Expr {
-  ParmVarDecl *Param;
+  /// \brief The parameter whose default is being used.
+  ///
+  /// When the bit is set, the subexpression is stored after the 
+  /// CXXDefaultArgExpr itself. When the bit is clear, the parameter's
+  /// actual default expression is the subexpression.
+  llvm::PointerIntPair<ParmVarDecl *, 1, bool> Param;
 
 protected:
   CXXDefaultArgExpr(StmtClass SC, ParmVarDecl *param)
-    : Expr(SC, param->hasUnparsedDefaultArg() ?
-           param->getType().getNonReferenceType()
-           : param->getDefaultArg()->getType()),
-    Param(param) { }
+    : Expr(SC, 
+           param->hasUnparsedDefaultArg()
+             ? param->getType().getNonReferenceType()
+             : param->getDefaultArg()->getType()),
+      Param(param, false) { }
 
+  CXXDefaultArgExpr(StmtClass SC, ParmVarDecl *param, Expr *SubExpr)
+    : Expr(SC, SubExpr->getType()), Param(param, true) 
+  {
+    *reinterpret_cast<Expr **>(this + 1) = SubExpr;
+  }
+  
+protected:
+  virtual void DoDestroy(ASTContext &C);
+  
 public:
   // Param is the parameter whose default argument is used by this
   // expression.
@@ -402,13 +417,26 @@
     return new (C) CXXDefaultArgExpr(CXXDefaultArgExprClass, Param);
   }
 
+  // Param is the parameter whose default argument is used by this
+  // expression, and SubExpr is the expression that will actually be used.
+  static CXXDefaultArgExpr *Create(ASTContext &C, ParmVarDecl *Param, 
+                                   Expr *SubExpr);
+  
   // Retrieve the parameter that the argument was created from.
-  const ParmVarDecl *getParam() const { return Param; }
-  ParmVarDecl *getParam() { return Param; }
+  const ParmVarDecl *getParam() const { return Param.getPointer(); }
+  ParmVarDecl *getParam() { return Param.getPointer(); }
 
   // Retrieve the actual argument to the function call.
-  const Expr *getExpr() const { return Param->getDefaultArg(); }
-  Expr *getExpr() { return Param->getDefaultArg(); }
+  const Expr *getExpr() const { 
+    if (Param.getInt())
+      return *reinterpret_cast<Expr const * const*> (this + 1);
+    return getParam()->getDefaultArg(); 
+  }
+  Expr *getExpr() { 
+    if (Param.getInt())
+      return *reinterpret_cast<Expr **> (this + 1);
+    return getParam()->getDefaultArg(); 
+  }
 
   virtual SourceRange getSourceRange() const {
     // Default argument expressions have no representation in the
diff --git a/lib/AST/ExprCXX.cpp b/lib/AST/ExprCXX.cpp
index 5444a77..e4538fd 100644
--- a/lib/AST/ExprCXX.cpp
+++ b/lib/AST/ExprCXX.cpp
@@ -352,6 +352,19 @@
   }
 }
 
+CXXDefaultArgExpr *
+CXXDefaultArgExpr::Create(ASTContext &C, ParmVarDecl *Param, Expr *SubExpr) {
+  void *Mem = C.Allocate(sizeof(CXXDefaultArgExpr) + sizeof(Stmt *));
+  return new (Mem) CXXDefaultArgExpr(CXXDefaultArgExprClass, Param, SubExpr);
+}
+
+void CXXDefaultArgExpr::DoDestroy(ASTContext &C) {
+  if (Param.getInt())
+    getExpr()->Destroy(C);
+  this->~CXXDefaultArgExpr();
+  C.Deallocate(this);
+}
+
 CXXTemporary *CXXTemporary::Create(ASTContext &C,
                                    const CXXDestructorDecl *Destructor) {
   return new (C) CXXTemporary(Destructor);
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 09e1d6f..c56eeef 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -3078,15 +3078,30 @@
       if (Result.isInvalid())
         return ExprError();
 
-      if (SetParamDefaultArgument(Param, move(Result),
-                                  /*FIXME:EqualLoc*/
-                                  UninstExpr->getSourceRange().getBegin()))
+      // Check the expression as an initializer for the parameter.
+      InitializedEntity Entity
+        = InitializedEntity::InitializeParameter(Param);
+      InitializationKind Kind
+        = InitializationKind::CreateCopy(Param->getLocation(),
+               /*FIXME:EqualLoc*/UninstExpr->getSourceRange().getBegin());
+      Expr *ResultE = Result.takeAs<Expr>();
+
+      InitializationSequence InitSeq(*this, Entity, Kind, &ResultE, 1);
+      Result = InitSeq.Perform(*this, Entity, Kind, 
+                               MultiExprArg(*this, (void**)&ResultE, 1));
+      if (Result.isInvalid())
         return ExprError();
+      
+      // Build the default argument expression.
+      return Owned(CXXDefaultArgExpr::Create(Context, Param,
+                                             Result.takeAs<Expr>()));
     }
 
     // If the default expression creates temporaries, we need to
     // push them to the current stack of expression temporaries so they'll
     // be properly destroyed.
+    // FIXME: We should really be rebuilding the default argument with new
+    // bound temporaries; see the comment in PR5810.
     for (unsigned i = 0, e = Param->getNumDefaultArgTemporaries(); i != e; ++i)
       ExprTemporaries.push_back(Param->getDefaultArgTemporary(i));
   }
diff --git a/test/SemaTemplate/default-expr-arguments.cpp b/test/SemaTemplate/default-expr-arguments.cpp
index 8518d7b..0635801 100644
--- a/test/SemaTemplate/default-expr-arguments.cpp
+++ b/test/SemaTemplate/default-expr-arguments.cpp
@@ -1,5 +1,4 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
-
 template<typename T>
 class C { C(int a0 = 0); };
 
@@ -144,3 +143,26 @@
   }
 }
 
+// PR5810
+namespace PR5810 {
+  template<typename T>
+  struct allocator {
+    allocator() { int a[sizeof(T) ? -1 : -1]; } // expected-error{{array size is negative}}
+  };
+  
+  template<typename T>
+  struct vector {
+    vector(const allocator<T>& = allocator<T>()) {} // expected-note{{instantiation of}}
+  };
+  
+  struct A { };
+  
+  template<typename>
+  void FilterVTs() {
+    vector<A> Result;
+  }
+  
+  void f() {
+    vector<A> Result;
+  }
+}