Don't use an unnamed (anonymous) namespace in a header

Reason

It is almost always a bug to mention an unnamed namespace in a header file.

Example

// file foo.h:
namespace
{
    const double x = 1.234;  // bad

    double foo(double y)     // bad
    {
        return y + x;
    }
}

namespace Foo
{
    const double x = 1.234; // good

    inline double foo(double y)        // good
    {
        return y + x;
    }
}

Enforcement

• Flag any use of an anonymous namespace in a header file.