User-defined literals, underscores, and global names

Question

In C++11, we get user-defined literals. The C++ standard has examples of these, such as:

long double operator \"\" _w(long double);

And it

Answer 1

What's in a name? 3 Basic concepts [basic] tells us:

4 A name is a use of an identifier (2.11), operator-function-id (13.5), literal-operator-id (13.5.8), conversion-function-id (12.3.2), or template-id (14.2) that denotes an entity or label (6.6.4, 6.1).

which we cross-reference with 13.5.8 User-defined literals [over.literal]:

literal-operator-id:
operator "" identifier

While the name of a literal operator involves an identifier, that identifier does not denote an entity. (Or it's a different identifier and different name that denotes another entity or label altogether.) As such the name of a literal operator never starts with an underscore.

Something like operator""__w is problematic but this is not new: int i__0; is reserved as well.