<variant>: P0608R3 Improving variant's converting constructor/assignment

stl/inc/variant

@@ -967,10 +967,24 @@ using _Variant_destroy_layer = conditional_t<conjunction_v<is_trivially_destruct
 #pragma warning(disable : 4365) // '%s': conversion from '%s' to '%s', signed/unsigned mismatch
 #pragma warning(disable : 5215) // '%s' a function parameter with volatile qualified type is deprecated in C++20
 #endif // __clang__
-template <size_t _Idx, class _Ty>
+struct _Variant_type_test { // build Ti x[] = {std::forward<T>(t)};
+    template <size_t _Idx, class _Type>
+    static constexpr auto _Construct_array(_Type(&&)[1]) -> _Meta_list<integral_constant<size_t, _Idx>, _Type>;
+
+    template <size_t _Idx, class _Ty, class _Type>
+    using type = decltype(_Construct_array<_Idx, _Type>({_STD declval<_Ty&&>()}));
+};
+
+template <size_t _Idx, class _Ty, class _Type>
+using _Variant_type_resolver = typename _Variant_type_test::template type<_Idx, _Ty, _Type>;
+
+template <size_t _Idx, class _Type>
 struct _Variant_init_single_overload {
-    using _FTy = _Meta_list<integral_constant<size_t, _Idx>, _Ty> (*)(_Ty);
-    operator _FTy();
+    template <class _Ty>
+    using _FTy = _Variant_type_resolver<_Idx, _Ty, _Type>;
+
+    template <class _Ty>
+    auto operator()(_Ty&&, _Type) -> _FTy<_Ty>;
 };
 
 template <class _Indices, class... _Types>
@@ -987,11 +1001,12 @@ template <class Enable, class _Ty, class... _Types>
 struct _Variant_init_helper {}; // failure case (has no member "type")
 
 template <class _Ty, class... _Types>
-struct _Variant_init_helper<void_t<decltype(_Variant_init_overload_set<_Types...>{}(_STD declval<_Ty>()))>, _Ty,
+struct _Variant_init_helper<
+    void_t<decltype(_Variant_init_overload_set<_Types...>{}(_STD declval<_Ty&&>(), _STD declval<_Ty>()))>, _Ty,
     _Types...> {
     // perform overload resolution to determine the unique alternative that should be initialized in
     // variant<_Types...> from an argument expression with type and value category _Ty
-    using type = decltype(_Variant_init_overload_set<_Types...>{}(_STD declval<_Ty>()));
+    using type = decltype(_Variant_init_overload_set<_Types...>{}(_STD declval<_Ty&&>(), _STD declval<_Ty>()));
 };
 
 template <class _Ty, class... _Types> // extract the type from _Variant_init_helper

tests/std/test.lst

@@ -249,6 +249,7 @@ tests\P0556R3_bit_integral_power_of_two_operations
 tests\P0586R2_integer_comparison
 tests\P0595R2_is_constant_evaluated
 tests\P0607R0_inline_variables
+tests\P0608R3_sane_variant_converting_constructor
 tests\P0616R0_using_move_in_numeric
 tests\P0631R8_numbers_math_constants
 tests\P0660R10_jthread_and_cv_any

tests/std/tests/P0608R3_sane_variant_converting_constructor/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_17_matrix.lst

tests/std/tests/P0608R3_sane_variant_converting_constructor/test.cpp

@@ -0,0 +1,153 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Also tests for P1957R2: Converting from T* to bool should be considered narrowing
+
+#include <assert.h>
+#include <bitset>
+#include <optional>
+#include <string>
+#include <type_traits>
+#include <variant>
+#include <vector>
+
+using namespace std;
+
+struct double_double {
+    double_double(double x) : x_(x) {}
+
+    double x_;
+};
+struct convertible_bool {
+    convertible_bool(bool) {}
+    ~convertible_bool() = default;
+
+    operator bool() {
+        return true;
+    }
+};
+
+// P0608R3 examples
+static_assert(is_constructible_v<variant<string, bool>, const char*>);
+static_assert(is_constructible_v<variant<string, bool>, string>);
+static_assert(is_constructible_v<variant<char, optional<char16_t>>, char16_t>);
+static_assert(is_constructible_v<variant<int, reference_wrapper<double>>, double&>);
+static_assert(is_constructible_v<variant<float, int>, char>);
+static_assert(is_constructible_v<variant<float, long int>, int>);
+static_assert(is_constructible_v<variant<float, long long int>, int>);
+static_assert(is_constructible_v<variant<float, long, double>, int>);
+static_assert(is_constructible_v<variant<float, vector<int>, long long int>, int>);
+static_assert(is_constructible_v<variant<float, int, long long int>, char>);
+
+static_assert(!is_constructible_v<variant<float>, int>);
+static_assert(!is_constructible_v<variant<float, vector<int>>, int>);
+static_assert(!is_constructible_v<variant<float, char>, int>);
+
+// P1957R2
+static_assert(is_constructible_v<variant<bool, int>, bool>);
+
+// more examples
+static_assert(is_constructible_v<variant<double_double>, double>);
+static_assert(is_constructible_v<variant<vector<vector<int>>, optional<int>, int>, int>);
+static_assert(is_constructible_v<variant<vector<vector<int>>, optional<int>>, int>);
+static_assert(is_constructible_v<variant<vector<int>, optional<int>, float>, int>);
+static_assert(is_constructible_v<variant<bool>, convertible_bool>);
+static_assert(is_constructible_v<variant<bool, int>, convertible_bool>);
+static_assert(is_constructible_v<variant<convertible_bool>, bool>);
+static_assert(is_constructible_v<variant<float, bool, convertible_bool>, convertible_bool>);
+static_assert(is_constructible_v<variant<float, bool, convertible_bool>, bool>);
+static_assert(is_constructible_v<variant<char, int>, bool>);
+
+static_assert(!is_constructible_v<variant<double_double>, int>);
+static_assert(!is_constructible_v<variant<float>, unsigned int>);
+static_assert(!is_constructible_v<variant<float, long int, long long int>, int>);
+
+void test_variant_constructor_P0608R3() {
+    // P0608R3 runtime checks
+    variant<string, bool> a = "abc"; // string
+    assert(a.index() == 0);
+    assert(get<0>(a) == string("abc"));
+
+    variant<char, optional<char16_t>> b = u'\u2043'; // optional<char16_t>
+    assert(b.index() == 1);
+    assert(get<optional<char16_t>>(b) == u'\u2043');
+
+    double c_data                             = 3.14;
+    variant<int, reference_wrapper<double>> c = c_data; // reference_wrapper<double>
+    assert(c.index() == 1);
+    assert(get<1>(c) == c_data);
+
+    using T1 = variant<float, int>;
+    T1 d;
+    assert(d.index() == 0);
+    d = 0; // int
+    assert(d.index() == 1);
+
+    using T2 = variant<float, long>;
+    T2 e;
+    assert(e.index() == 0);
+    e = 0; // long
+    assert(e.index() == 1);
+
+    variant<float, int> f = 'a'; // int
+    assert(f.index() == 1);
+    assert(get<int>(f) == 97);
+
+    variant<float, long> g = 0; // long
+    assert(g.index() == 1);
+
+    variant<float, long, double> h = 0; // long
+    assert(h.index() == 1);
+
+    variant<float, vector<int>, long long int> i = 0; // long long int
+    assert(i.index() == 2);
+
+    variant<float, int, long long int> j = 'a'; // int
+    assert(j.index() == 1);
+    assert(get<int>(j) == 97);
+}
+
+void test_variant_constructor_P1957R2() {
+    bitset<4> a_bitset("0101");
+    bool a_data          = a_bitset[2];
+    variant<bool, int> a = a_data; // bool
+    assert(a.index() == 0);
+    assert(get<0>(a));
+
+    bitset<4> b_bitset("0101");
+    variant<bool, int> b  = b_bitset[2]; // bool
+    variant<bool, int> b2 = b_bitset[1]; // bool
+    assert(b.index() == 0);
+    assert(get<0>(b));
+    assert(b2.index() == 0);
+    assert(!get<0>(b2));
+}
+
+void test_variant_constructor_more_tests() {
+    variant<char, int, float, bool, vector<bool>> a = true; // bool
+    assert(a.index() == 3);
+
+    variant<bool, int> b = convertible_bool{true}; // bool
+    assert(b.index() == 0);
+    assert(get<0>(b));
+
+    variant<char, int, bool> c = false; // bool
+    assert(c.index() == 2);
+
+    variant<float, bool, convertible_bool> d = convertible_bool{true}; // convertible_bool
+    assert(d.index() == 2);
+
+    variant<float, bool, convertible_bool> e = bool{}; // bool
+    assert(e.index() == 1);
+    assert(!get<1>(e));
+
+    variant<float, bool> f = convertible_bool{true}; // bool
+    assert(f.index() == 1);
+    assert(get<1>(f));
+}
+
+int main() {
+    test_variant_constructor_P0608R3();
+    test_variant_constructor_P1957R2();
+    test_variant_constructor_more_tests();
+}