Potentially bogus warning about anonymous types

Question

A while ago I encountered what I believed to be a bogus warning of the form "... uses the anonymous namespace" with GCC 4.8.

I posted http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54060 , and I was told there that:
1. The warning is meant to say "... uses an anonymous type" rather than "... uses the anonymous namespace".
2. Aside from the poor wording, the warning is not bogus because it points out a potential ODR violation.

I recently encountered the same warning in a different situation, but I don't see where the potential ODR violation is this time. I suspect there may not be one (and thus the warning really is bogus in this case), but I wanted to double-check before filing a bug.

Here is the code in question:

test.cpp:

#include "test.hpp"

test.hpp:

template 
struct base
{
    static const bool value = true;
};

template 
struct outer;

template 
struct outer
{
    template  
    struct inner : public base
    {
    };
};

bool foo()
{
    auto lambda = [](){};
    typedef decltype(lambda) Lambda;
    return outer::inner::value;
}

When compiling test.cpp, I get the following warning:

In file included from test.cpp:1:0:
test.hpp: In instantiation of 'struct outer::inner':
test.hpp:23:39:   required from here
test.hpp:14:12: warning: 'outer::inner' has a base 'base' whose type uses the anonymous namespace [enabled by default]
     struct inner : public base

Could someone please confirm whether there is an ODR violation here, and if so explain what it is?

Answer 1

There is if you include test.hpp in multiple translation units (and the compiler has to assume you will) because foo() is not declared inline. Do that and the warning goes away.

Comments

I see. Is there something about a nested class, template specialization, or a base class that causes that example to be an ODR violation but not this one (which GCC does not warn about):

template
struct inner
{
    static const bool value = true;
};

bool foo()
{
    auto lambda = [](){};
    typedef decltype(lambda) Lambda;
    return inner::value;
}