Why is my log in the std namespace?

In the code below, I define a trivial log function. In main I try not to call it; I call std::log. Nevertheless, my own log is called; and I see "log!" on screen. Does anyone know why? I use G++ 4.7 and clang++ 3.2.

#include <iostream>
#include <cmath>

double log(const double x) { std::cout << "log!\n"; return x; }

int main(int argc, char *argv[])
{
  std::log(3.14);
  return 0;
}

C++ Standard 17.6.1.2 paragraph 4 (emphasis mine):

Except as noted in Clauses 18 through 30 and Annex D, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in the C Standard library (1.2) or the C Unicode TR, as appropriate, as if by inclusion. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope (3.3.6) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (7.3.3).

g++ does it the latter way so that some of the same header files can be reused for C and C++. So g++ is allowed to declare and define double log(double) in the global namespace.

Section 17.6.4.3.3 paragraphs 3 and 4:

Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.

Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++"linkage, or as a name of namespace scope in the global namespace.

And up at the top of Section 17.6.4.3 paragraph 2:

If a program declares or defines a name in a context where it is reserved, other than as explicitly allowed by this Clause, its behavior is undefined.

You, on the other hand, may not declare or define ::log in any way.

It's too bad the g++ toolchain doesn't give you any error messages, though.

What happens, I expect, is that std::log simply delegates to ::log. Unfortunately, ::log only provides a float overload, and you kindly provide a double overload, making yours a better match. But I still don't see how it even gets considered in the overload set.

On libstdc++'s cmath you will see this:

using ::log;

So it's bringing in the math.h functions from the global namespace into std. Unfortunately you are supplying an implementation for double log(double), so the linker will not use the one from the math lib. So definitely a bug in libstdc++.

EDIT: I claim it's a bug in libstdc++ because std::log should not suffer from interferences with the C library when you are explicitly asking for the std:: versions. Of course, this way to override standard library functions is an old "feature" from the C language.

EDIT 2: I found out that the standard doesn't actually forbid bringing names from the global namespace into std. So not a bug after all, only a consequence of the implementation details.