https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283
Bug ID: 115283
Summary: [14 Regression] "used but never defined" with extern
templates
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at hazardy dot de
Target Milestone: ---
This reduced code does work with GCC 13, but with 14 I get the "used but never
defined" warning (and later undefined refernces, even tho the symbols are
emitted to the object files).
See https://godbolt.org/z/Gsz4x7d6h
#include <concepts>
#include <type_traits>
template<typename T>
class C
{
public:
using ResultT = std::conditional_t<std::same_as<T, void>, T, T>;
ResultT Result;
C(ResultT result);
};
template<typename T>
C<T>::C(ResultT result)
: Result{result}
{
return;
}
extern template class C<bool>;
void foo()
{
C<bool> r(true);
}
It is absolutely necessary to use the std::same_as for the conditional. If it
is replaced with true (or false) everything works. Even exchanging it with
sizeof(T) >= 2 works. std::is_same_v also works.
So maybe it's a bug in std::same_as?
