<ranges>: Fix piecewise construction for repeat_view (#5388)

frederick-vs-ja · web-flow · commit 7385267ea07c · 2025-04-10T15:58:44.000-07:00
diff --git a/stl/inc/ranges b/stl/inc/ranges
@@ -1063,6 +1063,20 @@ namespace ranges {
     } // namespace views
 
 #if _HAS_CXX23
+    template <class _Ty, class _Tuple, size_t... _Indices>
+    constexpr _Movable_box<_Ty> _Make_box_from_tuple_impl(_Tuple&& _Tpl, index_sequence<_Indices...>)
+        noexcept(is_nothrow_constructible_v<_Ty, decltype(_STD get<_Indices>(_STD forward<_Tuple>(_Tpl)))...>) {
+        return _Movable_box<_Ty>(in_place, _STD get<_Indices>(_STD forward<_Tuple>(_Tpl))...);
+    }
+
+    template <class _Ty, class _Tuple>
+    constexpr _Movable_box<_Ty> _Make_box_from_tuple(_Tuple&& _Tpl)
+        noexcept(noexcept(_RANGES _Make_box_from_tuple_impl<_Ty>(
+            _STD forward<_Tuple>(_Tpl), make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>{}))) {
+        return _RANGES _Make_box_from_tuple_impl<_Ty>(
+            _STD forward<_Tuple>(_Tpl), make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>{});
+    }
+
     template <class _Ty>
     concept _Integer_like_with_usable_difference_type =
         _Signed_integer_like<_Ty> || (_Integer_like<_Ty> && weakly_incrementable<_Ty>);
@@ -1229,11 +1243,6 @@ namespace ranges {
         /* [[no_unique_address]] */ _Movable_box<_Ty> _Value{};
         /* [[no_unique_address]] */ _Bo _Bound{};
 
-        template <class _Tuple, size_t... _Indices>
-        _NODISCARD_CTOR constexpr repeat_view(_Tuple& _Val, index_sequence<_Indices...>, _Bo _Bound_)
-            noexcept(is_nothrow_constructible_v<_Ty, tuple_element_t<_Indices, _Tuple>...>)
-            : _Value(in_place, _Tuple_get<_Indices>(_STD move(_Val))...), _Bound(_STD move(_Bound_)) {}
-
     public:
         repeat_view()
             requires default_initializable<_Ty>
@@ -1262,9 +1271,10 @@ namespace ranges {
             requires constructible_from<_Ty, _TArgs...> && constructible_from<_Bo, _BArgs...>
         _NODISCARD_CTOR constexpr explicit repeat_view(
             piecewise_construct_t, tuple<_TArgs...> _Val_args, tuple<_BArgs...> _Bound_args = tuple<>{})
-            noexcept(is_nothrow_constructible_v<_Ty, _TArgs...>
-                     && noexcept(_STD make_from_tuple<_Bo>(_Bound_args))) // strengthened
-            : repeat_view(_Val_args, index_sequence_for<_TArgs...>{}, _STD make_from_tuple<_Bo>(_Bound_args)) {
+            noexcept(noexcept(_RANGES _Make_box_from_tuple<_Ty>(_STD move(_Val_args)))
+                     && noexcept(_STD make_from_tuple<_Bo>(_STD move(_Bound_args)))) // strengthened
+            : _Value(_RANGES _Make_box_from_tuple<_Ty>(_STD move(_Val_args))),
+              _Bound(_STD make_from_tuple<_Bo>(_STD move(_Bound_args))) {
 #if _ITERATOR_DEBUG_LEVEL != 0
             if constexpr (_Signed_integer_like<_Bo>) {
                 _STL_VERIFY(_Bound >= 0, "Bound must be >= 0");
diff --git a/tests/std/tests/P2474R2_views_repeat/test.cpp b/tests/std/tests/P2474R2_views_repeat/test.cpp
@@ -288,6 +288,17 @@ struct tuple_tester {
     forward_tester z;
 };
 
+struct rvalue_only_to_sentinel {
+    template <class Sent>
+    constexpr operator Sent() && noexcept {
+        if constexpr (is_same_v<Sent, unreachable_sentinel_t>) {
+            return {};
+        } else {
+            return Sent{1};
+        }
+    }
+};
+
 
 template <class IntLike>
 constexpr void test_iterator_arithmetic() {
@@ -300,6 +311,51 @@ constexpr void test_iterator_arithmetic() {
     assert(last - first == 15);
 }
 
+template <class T>
+constexpr void test_piecewise_construction_from_empty_braces() {
+    ranges::repeat_view<T> r{piecewise_construct, {}, {}};
+    assert(r.front() == T{});
+}
+
+#ifndef _M_CEE // TRANSITION, VSO-1659496
+template <class T>
+struct holder {
+    T t;
+};
+
+struct incomplete;
+
+struct adl_validator {
+    constexpr explicit adl_validator(holder<incomplete>*) noexcept {}
+};
+
+constexpr void test_piecewise_construction_for_adl() {
+    constexpr holder<incomplete>* arg_ptr = nullptr;
+    // Intentionally std::-qualify make_tuple to avoid ADL.
+    {
+        (void) ranges::repeat_view<adl_validator>{piecewise_construct, std::make_tuple(arg_ptr)};
+    }
+    {
+        ranges::repeat_view<adl_validator, ptrdiff_t> r{piecewise_construct, std::make_tuple(arg_ptr)};
+        assert(r.size() == 0);
+    }
+    {
+        ranges::repeat_view<adl_validator, ptrdiff_t> r{
+            piecewise_construct, std::make_tuple(arg_ptr), std::make_tuple(ptrdiff_t{42})};
+        assert(r.size() == 42);
+    }
+}
+#endif // ^^^ no workaround ^^^
+
+template <class Sent>
+constexpr void test_make_sentinel_from_rvalue() {
+    ranges::repeat_view<int, Sent> r{piecewise_construct, make_tuple(42), make_tuple(rvalue_only_to_sentinel{})};
+    assert(r.front() == 42);
+    if constexpr (!is_same_v<Sent, unreachable_sentinel_t>) {
+        assert(r.size() == 1);
+    }
+}
+
 constexpr bool test() {
     using namespace string_literals;
 
@@ -356,6 +412,16 @@ constexpr bool test() {
     test_iterator_arithmetic<wchar_t>();
     test_iterator_arithmetic<_Signed128>();
 
+    // GH-5387: "<ranges>: Cannot construct repeat_view via {piecewise_construct, {}, {}}"
+    test_piecewise_construction_from_empty_braces<int>();
+    test_piecewise_construction_from_empty_braces<string>();
+#ifndef _M_CEE // TRANSITION, VSO-1659496
+    test_piecewise_construction_for_adl();
+#endif // ^^^ no workaround ^^^
+    test_make_sentinel_from_rvalue<int>();
+    test_make_sentinel_from_rvalue<_Signed128>();
+    test_make_sentinel_from_rvalue<unreachable_sentinel_t>();
+
     return true;
 }
 
