https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89855
--- Comment #6 from James Y Knight <foom at fuhm dot net> --- Someone has pointed out to me that the standard actually says "name", which I had internalized as meaning "declaration", but it doesn't. This arguably does make the GCC implementation non-compliant with the spec. >From http://eel.is/c++draft/headers#5 > Except as noted in [library] through [thread] and [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 [support] through [thread] and > [depr]) are first declared within the global namespace scope and are then > injected into namespace std by explicit using-declarations. I think that text implies that all of the overloads of a particular name must consistently either be defined in the global namespace (and then imported into std with a using-declaration) or defined in the std namespace.