C++11 Enum forward causes "underlying type mismatch"

I'm working in C++11 and including an h file implemented in C++03. In the h file I'm including there's an enum Foo defined. I want to declare a forward to it in code.h and use it in code.cpp:

header.h:

enum Foo {A=1};

code.h:

enum Foo : int; // also tried : unsigned int, long, short, unsigned short, char, unsigned char
void bar(Foo foo);

code.cpp:

#include header.h
void bar(Foo foo) { }

This is the error I get when I compile (tested g++ 4.8.5 and g++ 5.3.1):

In file included from code.cpp:2:0:
header.h:1:6: error: underlying type mismatch in enum ‘enum Foo’
 enum Foo {A=1};
      ^
In file included from code.cpp:1:0:
code.h:3:12: error: previous definition here
 enum Foo : int;

I can fix this error if I change header.h to:

enum Foo : int {A=1};

But I don't own that header and can't change it.