The <chrono> synopsis is in [time.syn] - there is no subclause [chrono.syn]. I have no explanation for why many of the names are qualified - e.g. struct common_type<chrono::duration in namespace std::chrono need not be qualified - other than that std::chrono was one of our first nested namespaces and LWG policy for how to specify components in sub-namespaces of std was less developed than today.

std::filesystem is way older than std::chrono, yet we still don't describe the "leave off namespaces" presentation form properly.

std::filesystem is way older than std::chrono, yet we still don't describe the "leave off namespaces" presentation form properly.

std::chrono was added in C++11, std::filesystem in C++17. The example I pointed out above is completely invalid, however, since it's a specialization of common_type in namespace std and therefore must qualify chrono::duration.

Let's start over: The user-defined literals in <chrono> are defined in namespace std::literals::chrono_literals, which is not a subnamespace of std::chrono, so the return types must be chrono::-qualified; they would not be found by unqualified lookup.

I apparently overlooked a closing brace from a previous "namespace chrono {".

So, I agree we need the "chrono::" qualification on the return type for the literal operators. Thanks for looking into this.