Notes

C++17 standard clause 20 section 5.1.2.4 says that:
Except as noted in Clauses [library] through [thread] and Annex [depr], the contents of each header cname is the same as that of the corresponding header name.h as specified in the C standard library. In the C++ standard library, however, the declarations (except for names which are defined as macros in C) are within namespace scope of the namespace std. It is unspecified whether these names (including any overloads added in Clauses [language.support] through [thread] and Annex [depr]) are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations.

So, standard allows <cxx> header to duplicate any function they declare in both namespace std and the global namespace. But why C++ standard doesn't mandate that every <cxx> header must put their contents in namespace std & not in global namespace. Isn't this better solution ? This behaviour seems very annoying. Consider the example from this question:
https://www.quora.com/Why-this-C++-program-that-uses-standard-library-function-code-abs-code-gives-different-outputs-on-different-compilers
This program exhibits different behavior on different compilers. Program also exhibits different behavior on different version of same compiler ( g++ & clang++ ) .