https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120190
Bug ID: 120190
Summary: Definition of the primary template of std::format_kind
seems IFNDR
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
Currently, libstd++ defines the primary template of std::format_kind as:
```
template<typename _Rg>
constexpr auto format_kind =
__primary_template_not_defined(
format_kind<_Rg> // you can specialize this for non-const input ranges
);
```
It seems that such definition is always invalid, and causes the program
ill-formed, no diagnostic required per [temp.res.general]/6
(https://eel.is/c++draft/temp.res.general#6).
Recently, Clang starts to reject such definition before instantiation
(https://godbolt.org/z/aYhd851hE).
IIUC we should use either static_assert or some merging trick to be
pedantically correct.
