[Accepted at the November, 2020 meeting as part of paper P1787R6 and moved to DR at the February, 2021 meeting.]

The current wording of 13.8.4 [temp.dep.res] seems to assume that dependent names can only appear in the definition of a template:

In resolving dependent names, names from the following sources are considered:

• Declarations that are visible at the point of definition of the template.

• Declarations from namespaces associated with the types of the function arguments both from the instantiation context (13.8.4.1 [temp.point]) and from the definition context.

However, dependent names can occur in non-defining declarations of the template as well; for instance,

    template<typename T>
    T foo(T, decltype(bar(T())));

bar needs to be looked up, even though there is no definition of foo in the translation unit.