https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112607
Bug ID: 112607
Summary: <format>: _Normalize does not consider char_type for
the basic_string_view case
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: hewillk at gmail dot com
Target Milestone: ---
When T in basic_format_arg(T& v) is a specialization of basic_string_view or
basic_string, format#arg-6.8 indicates:
otherwise, if TD is a specialization of basic_string_view or basic_string and
TD::value_type is char_type, initializes value with
basic_string_view<char_type>(v.data(), v.size());
We need to consider TD::value_type is char_type.
However, libstd++ only uses __is_specialization_of to detect whether T is a
specialization of basic_string_view or basic_string (format#L3118-L3121):
else if constexpr (__is_specialization_of<_Td, basic_string_view>)
return type_identity<basic_string_view<_CharT>>();
else if constexpr (__is_specialization_of<_Td, basic_string>)
return type_identity<basic_string_view<_CharT>>();
This causes basic_format_arg to incorrectly use wstring_view to initialize
string_view when customizing std::wstring.
https://godbolt.org/z/6Kd16z8qK
#include <format>
template<>
struct std::formatter<std::wstring> : std::formatter<std::string> {
auto format(const std::wstring& obj, auto& ctx) const {
return std::formatter<std::string>::format(" ", ctx);
}
};
int main(){
std::wstring wstr;
std::string str = std::format("{}", wstr);
}
