`<iomanip>`: ADL for the return types of `get_time`/`put_time` looks into the global namespace

[frederick-vs-ja]

Currently, the user-defined overload affects overload resolution. Godbolt link. [ext.manip] arguably disallows this.

#include <iostream>
#include <iomanip>
#include <ctime>

template<class CharT, class Traits, class T>
std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, T&&) {
    return os << "catch-all\n";
}

int main() {
    std::tm tmv{};
    std::cout << std::put_time(&tmv, "%Y-%M");
}

The reason is that put_time returns _Timeobj<_Elem, const tm*> whose associated namespaces contains the global namespace.

Perhaps the right fix would need to change the return type.

[vmichal]

BTW is seems that std::put_money has the same issue (and also possibly quoted, if I understood your standard reference correctly).

[StephanTLavavej]

_Monobj, _Quote_out, and _Quote_in should be unaffected because they don't involve struct tm.

_Timeobj is affected because of the wacky ADL rule that the namespaces of template arguments are considered.

[frederick-vs-ja]

_Monobj, _Quote_out, and _Quote_in should be unaffected because they don't involve struct tm.

Oops, it seems that these stuffs are also affected, because the global namespace can be an associated namespace of a basic_string specialization if one provides the allocator type in the global namespace. https://godbolt.org/z/Eazx18vjj

#include <iostream>
#include <iomanip>
#include <string>

template<class CharT, class Traits, class T>
std::basic_ostream<CharT, Traits>& operator<<(std::basic_ostream<CharT, Traits>& os, T&&) {
    return os << "catch-all\n";
}

template<class T>
struct my_ator : std::allocator<T> {};

using my_string = std::basic_string<char, std::char_traits<char>, my_ator<char>>;

int main() {
    std::cout << std::quoted(my_string{"42"});
}

[StephanTLavavej]

At this point I am inclined to say that the Standard should be worded to allow our obvious implementations. This usage is pathological.

[frederick-vs-ja]

Do we want to say the following? (looks in the opposite direction of WG21-P2538R1 to me)

For each implementation-defined or unspecified return type required in this document [Drafting note: The C++ Standard.],

• its associated namespaces may contain the global namespace, and
• if it is used as return type of a templated function that depends on a program-defined class or enumeration type, its associated namespaces may also contain associated namespaces of that program-defined type.

And for each operation on such a type specified in this document that involves argument-dependent lookup, if a program-defined non-member overload is selected, the program is ill-formed, no diagnostic required.