Given the statements below (emphasis mine) in §3.5/4 and in the Note [94] in §7.3.1.1/1, I'd like to have one single example of an entity declared in a unnamed namespace that has external linkage.
§3.5/4
An unnamed namespace or a namespace declared directly or indirectly within an unnamed namespace has internal linkage. All other namespaces have external linkage. A name having namespace scope that has not been given internal linkage above has the same linkage as the enclosing namespace if it is the name of
- a variable; or
- a function; or
- a named class (Clause 9), or an unnamed class defined in a typedef declaration in which the class has the typedef name for linkage purposes (7.1.3); or
- a named enumeration (7.2), or an unnamed enumeration defined in a typedef declaration in which the enumeration has the typedef name for linkage purposes (7.1.3); or
- an enumerator belonging to an enumeration with linkage; or
- a template.
Note [94] on §7.3.1.1/1:
Although entities in an unnamed namespace might have external linkage, they are effectively qualified by a name unique to their translation unit and therefore can never be seen from any other translation unit.
You are looking at a defect in the standard.
The change that makes unnamed namespace members have internal linkage happened fairly late in the C++11 standardization process, in November 2010 (CWG issue 1113). As a result, a number of places in the standard needs to be changed, but weren't. One of which is the footnote you quoted.
CWG issue 1603, currently in "ready" status (read: the resolution is likely to be adopted at the next committee meeting), will fix this and a number of other issues related to giving internal linkage to unnamed namespace members.