[vector] Add specialization of fill_n

Addresses #625

Author: miscco

stl/inc/xutility

@@ -4596,7 +4596,11 @@ _CONSTEXPR20 _OutIt fill_n(_OutIt _Dest, const _Diff _Count_raw, const _Ty& _Val
     _Algorithm_int_t<_Diff> _Count = _Count_raw;
     if (0 < _Count) {
         auto _UDest = _Get_unwrapped_n(_Dest, _Count);
-        if constexpr (_Fill_memset_is_safe<decltype(_UDest), _Ty>) {
+        if constexpr (_Is_vb_iterator<_OutIt>) {
+            const auto _Last = _Dest + static_cast<typename _OutIt::difference_type>(_Count);
+            _Fill_vbool(_Dest, _Last, _Val);
+            return _Last;
+        } else if constexpr (_Fill_memset_is_safe<decltype(_UDest), _Ty>) {
 #ifdef __cpp_lib_is_constant_evaluated
             if (!_STD is_constant_evaluated())
 #endif // __cpp_lib_is_constant_evaluated

tests/std/tests/GH_000625_vector_bool_optimization/test.cpp

@@ -24,6 +24,19 @@ void test_fill_helper(const size_t length) {
         vector<bool> dest_false(length + 3, true);
         fill(dest_false.begin(), prev(dest_false.end(), 3), false);
         assert(dest_false == result_false);
+
+        vector<bool> result_true_n(length, true);
+        result_true_n.resize(length + 3, false);
+        vector<bool> dest_true_n(length + 3, false);
+        const auto res_fill_n = fill_n(dest_true_n.begin(), length, true);
+        assert(dest_true_n == result_true_n);
+        assert(res_fill_n == prev(dest_true_n.end(), 3));
+
+        vector<bool> result_false_n(length, false);
+        result_false_n.resize(length + 3, true);
+        vector<bool> dest_false_n(length + 3, true);
+        fill_n(dest_false_n.begin(), length, false);
+        assert(dest_false_n == result_false_n);
     }
 
     // With offset
@@ -41,6 +54,21 @@ void test_fill_helper(const size_t length) {
         vector<bool> dest_false(length + 4, true);
         fill(next(dest_false.begin()), prev(dest_false.end(), 3), false);
         assert(dest_false == result_false);
+
+        vector<bool> result_true_n(length, true);
+        result_true_n.resize(length + 3, false);
+        result_true_n.insert(result_true_n.begin(), false);
+        vector<bool> dest_true_n(length + 4, false);
+        const auto res_fill_n = fill_n(next(dest_true_n.begin()), length, true);
+        assert(dest_true_n == result_true_n);
+        assert(res_fill_n == prev(dest_true_n.end(), 3));
+
+        vector<bool> result_false_n(length, false);
+        result_false_n.resize(length + 3, true);
+        result_false_n.insert(result_false_n.begin(), true);
+        vector<bool> dest_false_n(length + 4, true);
+        fill_n(next(dest_false_n.begin()), length, false);
+        assert(dest_false_n == result_false_n);
     }
 }