https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82478
Alexander Zaitsev <zamazan4ik at tut dot by> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |zamazan4ik at tut dot by --- Comment #6 from Alexander Zaitsev <zamazan4ik at tut dot by> --- One more problematic case: #include <type_traits> class A { int a; friend class B; }; class B { public: template <class T, class = void> struct trait: std::false_type {}; template <class T> struct trait<T, std::void_t<decltype(std::declval<T&>().a)>>: std::true_type {}; }; int main() { static_assert(B::trait<A>{}); return 0; } On clang(trunk) this code works well. On gcc(trunk) we have: prog.cc: In function 'int main()': prog.cc:15:61: error: 'int A::a' is private within this context struct trait<T, std::void_t<decltype(std::declval<T&>().a)>>: std::true_type {}; ~~~~~~~~~~~~~~~~~~~^ prog.cc:4:9: note: declared private here int a;