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From: "gcc at hazardy dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/115283] New: [14 Regression] "used but never defined" with
 extern templates
Date: Wed, 29 May 2024 21:36:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115283

            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 ne=
ver
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 =3D 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) >=3D 2 works. std::is_same_v also works.

So maybe it's a bug in std::same_as?=