P2162R2 Inheriting From variant (#2006)

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

stl/inc/yvals_core.h

@@ -111,6 +111,7 @@
 // P0858R0 Constexpr Iterator Requirements
 // P1065R2 constexpr INVOKE
 //     (the std::invoke function only; other components like bind and reference_wrapper are C++20 only)
+// P2162R2 Inheriting From variant
 
 // _HAS_CXX17 indirectly controls:
 // N4190 Removing auto_ptr, random_shuffle(), And Old <functional> Stuff
@@ -1198,7 +1199,7 @@
 #define __cpp_lib_shared_ptr_weak_type  201606L
 #define __cpp_lib_string_view           201803L
 #define __cpp_lib_to_chars              201611L
-#define __cpp_lib_variant               201606L
+#define __cpp_lib_variant               202102L
 #endif // _HAS_CXX17
 
 // C++20

tests/libcxx/expected_results.txt

@@ -654,6 +654,10 @@ std/language.support/support.limits/support.limits.general/chrono.version.pass.c
 # "should not be defined when TEST_HAS_BUILTIN(__builtin_addressof) || TEST_GCC_VER >= 700 is not defined!"
 std/language.support/support.limits/support.limits.general/memory.version.pass.cpp FAIL
 
+# Test should be enabled after LLVM update
+# updated test https://github.com/llvm/llvm-project/commit/0324b46cd873abc4fabe19f4bd468d10398ffd0d#diff-a9be5c3f8e18be99f40fb35f58960f400413a76252d86b53f80d76cb09cb53ef
+# should work
+std/language.support/support.limits/support.limits.general/variant.version.pass.cpp FAIL
 
 # *** LIKELY STL BUGS ***
 # Not yet analyzed, likely STL bugs. Assertions and other runtime failures.

tests/libcxx/skipped_tests.txt

@@ -654,6 +654,10 @@ language.support\support.limits\support.limits.general\chrono.version.pass.cpp
 # "should not be defined when TEST_HAS_BUILTIN(__builtin_addressof) || TEST_GCC_VER >= 700 is not defined!"
 language.support\support.limits\support.limits.general\memory.version.pass.cpp
 
+# Test should be enabled after LLVM update
+# updated test https://github.com/llvm/llvm-project/commit/0324b46cd873abc4fabe19f4bd468d10398ffd0d#diff-a9be5c3f8e18be99f40fb35f58960f400413a76252d86b53f80d76cb09cb53ef
+# should work
+language.support\support.limits\support.limits.general\variant.version.pass.cpp
 
 # *** LIKELY STL BUGS ***
 # Not yet analyzed, likely STL bugs. Assertions and other runtime failures.

tests/std/test.lst

@@ -420,6 +420,7 @@ tests\P1502R1_standard_library_header_units
 tests\P1614R2_spaceship
 tests\P1645R1_constexpr_numeric
 tests\P1682R3_to_underlying
+tests\P2162R2_std_visit_for_derived_classes_from_variant
 tests\VSO_0000000_allocator_propagation
 tests\VSO_0000000_any_calling_conventions
 tests\VSO_0000000_c_math_functions

tests/std/tests/P2162R2_std_visit_for_derived_classes_from_variant/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/P2162R2_std_visit_for_derived_classes_from_variant/test.cpp

@@ -0,0 +1,84 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <memory>
+#include <utility>
+#include <variant>
+
+using namespace std;
+
+struct Disconnected {
+    int val;
+};
+
+struct Connecting {
+    char val;
+};
+
+struct Connected {
+    double val;
+};
+
+struct State : variant<Disconnected, Connecting, Connected> {
+    using variant::variant;
+};
+
+void example1_from_p2162r2() {
+    State v1       = Disconnected{45};
+    const State v2 = Connecting{'d'};
+    visit([](auto x) { assert(x.val == 45); }, v1);
+    visit([](auto x) { assert(x.val == 'd'); }, v2);
+    visit([](auto x) { assert(x.val == 5.5); }, State{Connected{5.5}});
+    visit([](auto x) { assert(x.val == 45); }, move(v1));
+    visit([](auto x) { assert(x.val == 'd'); }, move(v2));
+}
+
+struct Expr;
+
+struct Neg {
+    shared_ptr<Expr> expr;
+};
+
+struct Add {
+    shared_ptr<Expr> lhs;
+    shared_ptr<Expr> rhs;
+};
+
+struct Mul {
+    shared_ptr<Expr> lhs;
+    shared_ptr<Expr> rhs;
+};
+
+struct Expr : variant<int, Neg, Add, Mul> {
+    using variant::variant;
+};
+
+int eval(const Expr& expr) {
+    struct visitor {
+        int operator()(int i) const {
+            return i;
+        }
+        int operator()(const Neg& n) const {
+            return -eval(*n.expr);
+        }
+        int operator()(const Add& a) const {
+            return eval(*a.lhs) + eval(*a.rhs);
+        }
+        int operator()(const Mul& m) const {
+            return eval(*m.lhs) * eval(*m.rhs);
+        }
+    };
+    return visit(visitor{}, expr);
+}
+
+void example2_from_p2162r2() {
+    // (1) + (2*3)
+    const Expr e = Add{make_shared<Expr>(1), make_shared<Expr>(Mul{make_shared<Expr>(2), make_shared<Expr>(3)})};
+    assert(eval(e) == (1 + 2 * 3));
+}
+
+int main() {
+    example1_from_p2162r2();
+    example2_from_p2162r2();
+}

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -1664,10 +1664,10 @@ STATIC_ASSERT(__cpp_lib_unwrap_ref == 201811L);
 #if _HAS_CXX17
 #ifndef __cpp_lib_variant
 #error __cpp_lib_variant is not defined
-#elif __cpp_lib_variant != 201606L
-#error __cpp_lib_variant is not 201606L
+#elif __cpp_lib_variant != 202102L
+#error __cpp_lib_variant is not 202102L
 #else
-STATIC_ASSERT(__cpp_lib_variant == 201606L);
+STATIC_ASSERT(__cpp_lib_variant == 202102L);
 #endif
 #else
 #ifdef __cpp_lib_variant