The __cpp_lib_ranges macro is missing from <algorithm>.
libstdc++-v3/ChangeLog:
* include/std/algorithm: Define __glibcxx_want_ranges.
* testsuite/25_algorithms/headers/algorithm/synopsis.cc: Check
feature test macro in C++20 mode.
#define __glibcxx_want_constexpr_algorithms
#define __glibcxx_want_freestanding_algorithm
#define __glibcxx_want_parallel_algorithm
+#define __glibcxx_want_ranges
#define __glibcxx_want_ranges_contains
#define __glibcxx_want_ranges_find_last
#define __glibcxx_want_ranges_fold
#include <algorithm>
+#if __cplusplus >= 202002L
+#ifndef __cpp_lib_ranges
+# error "Feature test macro for ranges is missing in <algorithm>"
+#elif __cpp_lib_ranges < 201911L
+# error "Feature test macro for ranges has wrong value in <algorithm>"
+#endif
+#endif
+
namespace std
{
// 25.1, non-modifying sequence operations: