LWG-3211 std::tuple<> should be trivially constructible (#1460)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/tuple

@@ -204,7 +204,7 @@ class tuple;
 template <>
 class tuple<> { // empty tuple
 public:
-    constexpr tuple() noexcept /* strengthened */ {}
+    constexpr tuple() noexcept = default; /* strengthened */
 
     constexpr tuple(const tuple&) noexcept /* strengthened */ {} // TRANSITION, ABI: should be defaulted
 

tests/std/tests/Dev11_0000000_tuple_cat/test.cpp

@@ -209,6 +209,9 @@ int main() {
         assert(make_from_tuple<int>(tuple<>{}) == 0);
 #endif // _HAS_CXX17
     }
+
+    // LWG-3211 std::tuple<> should be trivially constructible
+    STATIC_ASSERT(is_trivially_default_constructible_v<tuple<>>);
 }
 
 // Also test DevDiv-1205400 "C++ compiler: static_assert in std::tuple_element prevents SFINAE".