https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91953
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Reduced testcase: template<typename _Tp, _Tp __v> struct integral_constant { static constexpr _Tp value = __v; typedef _Tp value_type; typedef integral_constant<_Tp, __v> type; constexpr operator value_type() const noexcept { return value; } constexpr value_type operator()() const noexcept { return value; } }; auto f = [](auto value) { constexpr auto i = value; return i; }; void foo () { f (integral_constant<int, 0>{}); } So, shall value be constexpr because integral_constant is an empty class? This boils down to struct S {}; void foo (S s) { constexpr S x = s; } which GCC 7 as well as clang accepted, but GCC 8+ rejects.