Why overloaded operator== for std::weak_ptr instantiated with type defined in namespace can't be found?

I'm using Visual Studio 2015.

Any idea why this code compiles:

#include <memory>

class Foo;
class Bar;
typedef std::pair<Foo*,std::weak_ptr<Bar>> Object;
typedef std::vector<Object> ObjectVect;

bool operator==( std::weak_ptr<Bar> left,
                 std::weak_ptr<Bar> right )
{
    return left.lock() == right.lock();
}

int main( int argc, char* argv[] )
{
    ObjectVect vect;
    Object obj;
    auto foundIter = std::find( vect.begin(), vect.end(), obj );
    return 0;
}

while this one gives me error:

#include <memory>

class Foo;
namespace MyNamespace
{
    class Bar;
}
typedef std::pair<Foo*,std::weak_ptr<MyNamespace::Bar>> Object;
typedef std::vector<Object> ObjectVect;

bool operator==( std::weak_ptr<MyNamespace::Bar> left,
                 std::weak_ptr<MyNamespace::Bar> right )
{
    return left.lock() == right.lock();
}

int main( int argc, char* argv[] )
{
    ObjectVect vect;
    Object obj;
    auto foundIter = std::find( vect.begin(), vect.end(), obj );
    return 0;
}

Error C2678 binary '==': no operator found which takes a left-hand operand of type 'const std::weak_ptr' (or there is no acceptable conversion) test_cppunit_interpreter_base_multi_output c:\program files (x86)\microsoft visual studio 14.0\vc\include\utility 216

Looks like it failes to find the comparator when Bar is located in a namespace...

Am I doing something wrong? Or could this be a compiler bug?