Implement LWG-4035 single_view should provide empty (#4525)

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

stl/inc/ranges

@@ -1031,6 +1031,10 @@ namespace ranges {
             return data() + 1;
         }
 
+        _NODISCARD static constexpr bool empty() noexcept {
+            return false;
+        }
+
         _NODISCARD static constexpr size_t size() noexcept {
             return 1;
         }

tests/std/tests/P0896R4_views_single/test.cpp

@@ -52,6 +52,13 @@ constexpr bool test_one_type(T value, Args&&... args) {
     same_as<R> auto r0 = ranges::single_view{value};
     const R& cr0       = r0;
 
+    // validate member empty
+    static_assert(same_as<decltype(R::empty()), bool>);
+    static_assert(R::empty() == false);
+    static_assert(noexcept(R::empty()));
+    static_assert(noexcept(ranges::empty(r0)));
+    static_assert(noexcept(ranges::empty(cr0)));
+
     // validate member size
     static_assert(same_as<decltype(R::size()), size_t>);
     static_assert(R::size() == 1);