Why did most implementations still have cmath functions in global namespace in C++03?

Question

As I understand it, in C++03 #include <cmath> must declare the functions only in namespace std. Since C++11 they may additionally be declared in global namespace. This is the result of the practice where most C++ implementations declared the functions in global namespace (presumably #includeing <math.h>), and then just did using ::acos; etc. in namespace std.

But it seems to me that it would be similarly easy for the implementations to do something like this in their <cmath>:

namespace __C_LANGUAGE_MATH_H
{
#include <math.h>
}
// ...
namespace std
{
// ...
using __C_LANGUAGE_MATH_H::acos;
// ...
}

Why wasn't this practiced instead of just polluting the global namespace? Does my suggested solution have some major drawbacks which made the C++ committee allow pollution of global namespace in C++11?

NOTE: this does work, and linker gives no errors, at least with GCC + Binutils-ld. I've actually tried and edited GCC's cmath file like follows, and compiled my project which actively uses cmath functions successfully (after fixing some calls which mistakenly didn't specify std:: in the project):

mv /usr/include/c++/5.3.0/cmath{,.bak}
sed -i -e 's@\(# *include <math.h>\)@namespace __C_LANGUAGE_MATH_H\n{\n\1\n}@' \
    -e 's@\(using \+\)::@\1__C_LANGUAGE_MATH_H::@' /usr/include/c++/5.3.0/cmath

Answer 1

As stated in corresponding issue discussion in gcc bugzilla this approach will not allow including C header after C++ because of include guards:

#include <cmath>
#include <math.h> // include skipped by include guards

...
sin(x); // error: no sin in global namespace

As you mentioned standard requirements to C library wrappers were changed after issue was posted in defect report and previous requirements have been declared impractical from implementation point of view.