initial fill algo

stl/inc/algorithm

@@ -2388,6 +2388,41 @@ _FwdIt2 replace_copy_if(_ExPo&&, _FwdIt1 _First, _FwdIt1 _Last, _FwdIt2 _Dest, _
 }
 #endif // _HAS_CXX17
 
+#ifdef __cpp_lib_concepts
+namespace ranges {
+    template <class _FwdIt, class _T>
+
+    // VARIABLE ranges::fill
+    class _Fill_fn : private _Not_quite_object {
+    public:
+        using _Not_quite_object::_Not_quite_object;
+
+        // clang-format off
+            template <class _T, output_iterator<const _T&> _O, sentinel_for<_O> _S>
+            constexpr _O operator()(_O _First, _S _Last, const _T& _Value) const {
+                _Adl_verify_range(_First, _Last);
+                auto _UFirst = _Get_unwrapped(_STD move(_First));
+                auto _ULast = _Get_unwrapped(_STD move(_Last));
+                while (_UFirst != _ULast) {
+                    *_UFirst = _Value;
+                    ++_UFirst;
+                }
+
+                _Seek_wrapped(_First, _STD move(_UFirst));
+                return _First;
+            }
+
+            template <class _T, output_range<const _T&> _Rng>
+            constexpr borrowed_iterator_t<_Rng> operator()(_Rng&& _Range, const _T& _Value) const {
+                return (*this)(_RANGES begin(_Range), _RANGES end(_Range), _Value)
+            }
+        // clang-format on
+    };
+
+    inline constexpr _Fill_fn fill{_Not_quite_object::_Construct_tag{}};
+} // namespace ranges
+#endif // __cpp_lib_concepts
+
 // FUNCTION TEMPLATE generate
 template <class _FwdIt, class _Fn>
 _CONSTEXPR20 void generate(_FwdIt _First, _FwdIt _Last, _Fn _Func) { // replace [_First, _Last) with _Func()

tests/std/test.lst

@@ -241,6 +241,7 @@ tests\P0896R4_ranges_alg_copy_n
 tests\P0896R4_ranges_alg_count
 tests\P0896R4_ranges_alg_count_if
 tests\P0896R4_ranges_alg_equal
+tests\P0896R4_ranges_alg_fill
 tests\P0896R4_ranges_alg_find
 tests\P0896R4_ranges_alg_find_end
 tests\P0896R4_ranges_alg_find_first_of

tests/std/tests/P0896R4_ranges_alg_fill/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_matrix.lst

tests/std/tests/P0896R4_ranges_alg_fill/test.cpp

@@ -0,0 +1,39 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <cassert>
+#include <concepts>
+#include <ranges>
+#include <utility>
+#include <range_algorithm_support.hpp>
+
+constexpr void smoke_test() {
+    using ranges::fill, ranges::iterator_t;
+    using std::same_as;
+
+    int output[] = {13, 42, 1367};
+    {
+        const int value = 7;
+        auto result = fill(output.begin(), output.end(), value);
+        for (int i = 0; i < 3; ++i) {
+            assert(output[i] == 7);
+        }
+        assert(result == output.end());
+    }
+}
+
+int main() {
+    // STATIC_ASSERT((smoke_test(), true));
+    smoke_test();
+}
+
+struct instantiator {
+    template <class In>
+    static void call(In&& in = {}) {
+        (void) ranges::fill(in, SOMETHING);
+        (void) ranges::fill(ranges::begin(in), ranges::end(in), SOMETHING);
+    }
+};
+
+template void test_in_out<instantiator>();