https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82047
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Because std::complex has a default constructor that initializes all its
members. If you replace S with a type that leaves some members uninitialized
when default-initialized, it fails:
template<typename T> struct S {};
template<typename T> constexpr T v;
constexpr auto c = v<S<void>>; // ok
struct init { int i = 0; };
constexpr auto ok = v<init>; // ok
struct no_init { int i; };
constexpr auto fails = v<no_init>; // error
This is consistent with:
constexpr init x; // ok
constexpr no_init y; // error
constexpr no_init z{}; // ok
