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This implements LWG 4324, "unique_ptr<void>::operator* is not SFINAE-friendly", approved in Croydon, 2026. The noexcept-specifier added to C++23 by LWG 2762 is ill-formed if the pointer type cannot be dereferenced, which means that code which was checking whether the function exists (e.g. in a SFINAE context) no longer works. Such code was always questionable, because the function body was ill-formed if the pointer isn't dereferenceable, so the SFINAE check was probably giving the wrong answer, but it was possible to ask the question. Since LWG 2762 just asking the question can produce an error outside the immediate context, so operator* is no longer SFINAE-friendly. LWG 4324 adds a constraint to the function, so that it doesn't participate in overload resolution if it would be ill-formed. That's easy to implement for C++20 because we can just add a requires-clause. For C++11/14/17 we can't constrain it easily, so just adjust the noexcept-specifier so that it's not ill-formed. This still means you get the wrong answer (i.e. it looks like unique_ptr<void>::operator* is callable) but there's no error outside the immediate context. This restores the original semantics before the LWG 2762 change, for better or worse. libstdc++-v3/ChangeLog: * include/bits/unique_ptr.h (unique_ptr::_Nothrow_deref): New helper for pre-C++20. (unique_ptr::operator*): Either constrain or use _Nothrow_deref. * testsuite/20_util/unique_ptr/lwg4324.cc: New test. Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
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