Instance of unnamed class in unnamed namespace

Question

Using functors with parameters generally looks like that:

// Definition:
struct some_functor_with_params
{
    int ref;
    explicit some_functor_with_params(int ref) : ref(ref) {}
    bool operator ()(int i) const {return i == ref;}
};

// Usage:
std::vector<int> v;
std::find_if(v.begin(), v.end(), some_functor_with_params(42));

For a functor without parameters, the code may become

// Definition:
struct some_functor_without_params
{
    bool operator ()(int i) const {return i == 42;}
};

// Usage:
std::vector<int> v;
std::find_if(v.begin(), v.end(), some_functor_without_params());

but I would prefer the following usage:

std::vector<int> v;
std::find_if(v.begin(), v.end(), some_functor_without_params); // no parens

which has the following advantages:

• more concise
• more readable when invoking the functor directly: some_functor_without_params(i) rather than some_functor_without_params()(i)
• interchangeable with a function: bool some_functor_with_params(int i) {return i == 42;}

I implemented it the following way in a header file:

namespace {
    struct
    {
        bool operator ()(int i) const {return i == 42;}
    } some_functor_without_params;
}

I think the struct does not need a name since it has no user-declared constructor (nor destructor, nor anything requiring the struct name). I put the object in an unnamed namespace so that each compilation unit has its own some_functor_without_params and there is no 'double definition' link error.

Is there any performance penalty or any other drawback I cannot see?

This approach worked as expected until I encountered a very strange compilation error with Visual C++ 2013 which disappeared when naming the functor type, i.e. replacing

struct
{
    bool operator ()(int i) const {return i == 42;}
} some_functor_without_params;

with

struct some_functor_without_params_t
{
    bool operator ()(int i) const {return i == 42;}
} some_functor_without_params;

The error occurs only in Debug, not in Release, and states

error C2039: '<unnamed-type-some_functor_without_param>': is not a member of 'my_namespace:: ?? A0xbf2cc73f'

in file xstring, here:

_Myt& operator+=(const _Elem *_Ptr)
    {   // append [_Ptr, <null>) // <-- on this line(!)
    return (append(_Ptr));
    }

It looks like a compiler bug, what do you think?