Additional note (February, 2022):

The quoted paragraph was removed by P1103R3 (Merging Modules), approved 2019-02.

C++11 expanded the lookup rules for dependent function calls (13.8.4.2 [temp.dep.candidate] bullet 1.2) to include functions with internal linkage; previously only functions with external linkage were considered. However, 13.8.4.1 [temp.point] paragraph 6 still says,

The instantiation context of an expression that depends on the template arguments is the set of declarations with external linkage declared prior to the point of instantiation of the template specialization in the same translation unit.

Presumably this wording was overlooked and should be harmonized with the new specification.