gcc16 non-constant condition for static assertion

[finjulhich]

opencode found this explanation for the issue I described here https://codesynthesis.com/pipermail/odb-users/2026-May/005186.html

The virtual member pragma with get(...)/set(...) triggers GCC 16's new _Hashtable_enable_default_ctor check when the accessor functions are only declared (not defined inline). ODB's type-discovery compilation phase can't resolve is_default_constructible<equal_to<selector>> in its injected code context without inline definitions visible.

Bear with me please and let me know what more information I should add here ( while trying to a single file self contained reproducible error, the error never happened)

[boris-kolpackov]

opencode found this explanation for the issue

Doesn't make any sense to me. I still think the problem is as described in the GCC bug 86441.

We've run into it in our own use of ODB and one workaround is to manually per-instantiate the templates that ODB is unable to properly instantiate later (per the GCC bug report). Try adding something like this at the end of your example:

struct selector {
  std::string ctype;
  uint64_t    cid;
};
using things_t =  std::unordered_map<selector, double>;

#ifdef ODB_COMPILER
namespace std
{
  template class unordered_map<::selector, double>;
}
#endif