libstdc++-v3/testsuite/20_util/variant/visit_inherited.cc

// { dg-do compile { target c++17 } }
// { dg-add-options no_pch }

#include <variant>

// P2062R2 Inheriting from std::variant (resolving LWG 3052)

#if __cpp_lib_variant < 202102L
#error __cpp_lib_variant has the wrong value in <variant>
#endif

struct V : std::variant<int> {
    using std::variant<int>::variant;
};

constexpr int
test01()
{
  V v = 42;
  return std::visit([](int&){ return 17; }, v);
}
static_assert( test01() == 17 );

constexpr int
test02()
{
  const V c = 77;
  std::variant<char*, long> x = 88L;
  return std::visit([](auto&& a, auto&& b) {
    if constexpr (std::is_same_v<decltype(a), const int&&>)
      if constexpr (std::is_same_v<decltype(b), long&&>)
	return 99;
    return 0;
  },
  std::move(c), std::move(x));
}
static_assert( test02() == 99 );