Implement LWG 2904.

Summary:
- Removed the move-constructibe requirement from copy-assignable.
- Updated `__assign_alt` such that we direct initialize if
  `_Tp` can be `nothrow`-constructible from `_Arg`, or `_Tp`'s
  move construction can throw. Otherwise, construct a temporary and move it.
- Updated the tests to remove the pre-LWG2904 path.

Depends on D32671.

Reviewers: EricWF, CaseyCarter

Reviewed By: EricWF

Differential Revision: https://reviews.llvm.org/D33965

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304891 91177308-0d34-0410-b5e6-96231b3b80d8

include/variant

@@ -358,7 +358,6 @@ struct __traits {
 
   static constexpr _Trait __copy_assignable_trait = __common_trait(
       {__copy_constructible_trait,
-       __move_constructible_trait,
        __trait<_Types, is_trivially_copy_assignable, is_copy_assignable>...});
 
   static constexpr _Trait __move_assignable_trait = __common_trait(
@@ -877,25 +876,24 @@ public:
   }
 
 protected:
-  template <bool _CopyAssign, size_t _Ip, class _Tp, class _Arg>
+  template <size_t _Ip, class _Tp, class _Arg>
   inline _LIBCPP_INLINE_VISIBILITY
-  void __assign_alt(__alt<_Ip, _Tp>& __a,
+  void __assign_alt(__alt<_Ip, _Tp>& __a, _Arg&& __arg) {
-                    _Arg&& __arg,
-                    bool_constant<_CopyAssign> __tag) {
     if (this->index() == _Ip) {
       __a.__value = _VSTD::forward<_Arg>(__arg);
     } else {
       struct {
         void operator()(true_type) const {
-          __this->__emplace<_Ip>(_Tp(_VSTD::forward<_Arg>(__arg)));
+          __this->__emplace<_Ip>(_VSTD::forward<_Arg>(__arg));
         }
         void operator()(false_type) const {
-          __this->__emplace<_Ip>(_VSTD::forward<_Arg>(__arg));
+          __this->__emplace<_Ip>(_Tp(_VSTD::forward<_Arg>(__arg)));
         }
         __assignment* __this;
         _Arg&& __arg;
       } __impl{this, _VSTD::forward<_Arg>(__arg)};
-      __impl(__tag);
+      __impl(bool_constant<is_nothrow_constructible_v<_Tp, _Arg> ||
+                           !is_nothrow_move_constructible_v<_Tp>>{});
     }
   }
 
@@ -912,8 +910,7 @@ protected:
           [this](auto& __this_alt, auto&& __that_alt) {
             this->__assign_alt(
                 __this_alt,
-                _VSTD::forward<decltype(__that_alt)>(__that_alt).__value,
+                _VSTD::forward<decltype(__that_alt)>(__that_alt).__value);
-                is_lvalue_reference<_That>{});
           },
           *this, _VSTD::forward<_That>(__that));
     }
@@ -1013,8 +1010,7 @@ public:
   inline _LIBCPP_INLINE_VISIBILITY
   void __assign(_Arg&& __arg) {
     this->__assign_alt(__access::__base::__get_alt<_Ip>(*this),
-                       _VSTD::forward<_Arg>(__arg),
+                       _VSTD::forward<_Arg>(__arg));
-                       false_type{});
   }
 
   inline _LIBCPP_INLINE_VISIBILITY
@@ -1088,7 +1084,6 @@ class _LIBCPP_TEMPLATE_VIS variant
           __all<is_move_constructible_v<_Types>...>::value>,
       private __sfinae_assign_base<
           __all<(is_copy_constructible_v<_Types> &&
-                 is_move_constructible_v<_Types> &&
                  is_copy_assignable_v<_Types>)...>::value,
           __all<(is_move_constructible_v<_Types> &&
                  is_move_assignable_v<_Types>)...>::value> {

test/std/utilities/variant/variant.variant/variant.assign/T.pass.cpp

@@ -199,12 +199,8 @@ void test_T_assignment_performs_construction() {
       assert(false);
     } catch (...) { /* ... */
     }
-#ifdef _LIBCPP_VERSION // LWG2904
-    assert(v.valueless_by_exception());
-#else // _LIBCPP_VERSION
     assert(v.index() == 0);
     assert(std::get<0>(v) == "hello");
-#endif // _LIBCPP_VERSION
   }
   {
     using V = std::variant<ThrowsAssignT, std::string>;
@@ -213,28 +209,6 @@ void test_T_assignment_performs_construction() {
     assert(v.index() == 0);
     assert(std::get<0>(v).value == 42);
   }
-#ifdef _LIBCPP_VERSION // LWG2904
-  {
-    // Test that nothrow direct construction is preferred to nothrow move.
-    using V = std::variant<MoveCrashes, std::string>;
-    V v(std::in_place_type<std::string>, "hello");
-    v = 42;
-    assert(v.index() == 0);
-    assert(std::get<0>(v).value == 42);
-  }
-  {
-    // Test that throwing direct construction is preferred to copy-and-move when
-    // move can throw.
-    using V = std::variant<ThrowsCtorTandMove, std::string>;
-    V v(std::in_place_type<std::string>, "hello");
-    try {
-      v = 42;
-      assert(false);
-    } catch(...) { /* ... */
-    }
-    assert(v.valueless_by_exception());
-  }
-#endif // _LIBCPP_VERSION // LWG2904
 #endif // TEST_HAS_NO_EXCEPTIONS
 }
 

test/std/utilities/variant/variant.variant/variant.assign/copy.pass.cpp

@@ -224,13 +224,7 @@ void test_copy_assignment_sfinae() {
   }
   {
     using V = std::variant<int, CopyOnly>;
-#ifdef _LIBCPP_VERSION // LWG2904
-    // variant only provides copy assignment when both the copy and move
-    // constructors are well formed
-    static_assert(!std::is_copy_assignable<V>::value, "");
-#else // _LIBCPP_VERSION // LWG2904
     static_assert(std::is_copy_assignable<V>::value, "");
-#endif // _LIBCPP_VERSION // LWG2904
   }
   {
     using V = std::variant<int, NoCopy>;
@@ -263,12 +257,10 @@ void test_copy_assignment_sfinae() {
     using V = std::variant<int, TCopyAssignNTMoveAssign>;
     static_assert(std::is_trivially_copy_assignable<V>::value, "");
   }
-#ifndef _LIBCPP_VERSION // LWG2904
   {
     using V = std::variant<int, CopyOnly>;
     static_assert(std::is_trivially_copy_assignable<V>::value, "");
   }
-#endif // _LIBCPP_VERSION
 }
 
 void test_copy_assignment_empty_empty() {
@@ -487,41 +479,21 @@ void test_copy_assignment_different_index() {
       assert(false);
     } catch (...) { /* ... */
     }
-#ifdef _LIBCPP_VERSION // LWG2904
-    // Test that if copy construction throws then original value is unchanged.
-    assert(v1.index() == 2);
-    assert(std::get<2>(v1) == "hello");
-#else // _LIBCPP_VERSION // LWG2904
     // Test that copy construction is used directly if move construction may throw,
     // resulting in a valueless variant if copy throws.
     assert(v1.valueless_by_exception());
-#endif // _LIBCPP_VERSION // LWG2904
   }
   {
     using V = std::variant<int, MoveThrows, std::string>;
     V v1(std::in_place_type<std::string>, "hello");
     V v2(std::in_place_type<MoveThrows>);
     assert(MoveThrows::alive == 1);
-#ifdef _LIBCPP_VERSION // LWG2904
-    // Test that if move construction throws then the variant is left
-    // valueless by exception.
-    try {
-      v1 = v2;
-      assert(false);
-    } catch (...) { /* ... */
-    }
-    assert(v1.valueless_by_exception());
-    assert(v2.index() == 1);
-    assert(MoveThrows::alive == 1);
-#else // _LIBCPP_VERSION // LWG2904
     // Test that copy construction is used directly if move construction may throw.
     v1 = v2;
     assert(v1.index() == 1);
     assert(v2.index() == 1);
     assert(MoveThrows::alive == 2);
-#endif // _LIBCPP_VERSION // LWG2904
   }
-#ifndef _LIBCPP_VERSION // LWG2904
   {
     // Test that direct copy construction is preferred when it cannot throw.
     using V = std::variant<int, CopyCannotThrow, std::string>;
@@ -533,7 +505,6 @@ void test_copy_assignment_different_index() {
     assert(v2.index() == 1);
     assert(CopyCannotThrow::alive == 2);
   }
-#endif // _LIBCPP_VERSION // LWG2904
   {
     using V = std::variant<int, CopyThrows, std::string>;
     V v1(std::in_place_type<CopyThrows>);

test/std/utilities/variant/variant.variant/variant.assign/move.pass.cpp

@@ -183,13 +183,7 @@ void test_move_assignment_sfinae() {
   }
   {
     using V = std::variant<int, CopyOnly>;
-#ifdef _LIBCPP_VERSION // LWG2904
-    // variant only provides move assignment when both the move constructor
-    // and move assignment operator are well formed.
-    static_assert(!std::is_move_assignable<V>::value, "");
-#else // _LIBCPP_VERSION // LWG2904
     static_assert(std::is_move_assignable<V>::value, "");
-#endif // _LIBCPP_VERSION // LWG2904
   }
   {
     using V = std::variant<int, NoCopy>;
@@ -232,12 +226,10 @@ void test_move_assignment_sfinae() {
     using V = std::variant<int, TrivialCopyNontrivialMove>;
     static_assert(!std::is_trivially_move_assignable<V>::value, "");
   }
-#ifndef _LIBCPP_VERSION // LWG2904
   {
     using V = std::variant<int, CopyOnly>;
     static_assert(std::is_trivially_move_assignable<V>::value, "");
   }
-#endif // _LIBCPP_VERSION // LWG2904
 }
 
 void test_move_assignment_empty_empty() {