[PATCH] PR libstdc++/85184 remove debug assertions from std::variant

[Jonathan Wakely]

The __glibcxx_assert macro should be used to check preconditions that
users must meet, not to check postconditions that the implementation
must meet. We have tests to verify std::variant meets its
postconditions, users shouldn't pay for those checks at runtime.

        PR libstdc++/85184
        * include/std/variant (_Copy_assign_base, _Move_assign_base, variant):
        Remove assertions.
        (variant::emplace<_Tp>): Remove result of emplace<N> directly.

Tested powerpc64le-linux, committed to trunk.



commit b54865992e3553dbf5aa93726996da3d3d891806
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Apr 3 10:28:33 2019 +0100

    PR libstdc++/85184 remove debug assertions from std::variant
    
    The __glibcxx_assert macro should be used to check preconditions that
    users must meet, not to check postconditions that the implementation
    must meet. We have tests to verify std::variant meets its
    postconditions, users shouldn't pay for those checks at runtime.
    
            PR libstdc++/85184
            * include/std/variant (_Copy_assign_base, _Move_assign_base, 
variant):
            Remove assertions.
            (variant::emplace<_Tp>): Remove result of emplace<N> directly.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index a7cc2065073..a21ef3005cf 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -612,7 +612,6 @@ namespace __variant
              this->_M_reset();
            return {};
          }, __variant_cast<_Types...>(__rhs));
-       __glibcxx_assert(this->_M_index == __rhs._M_index);
        return *this;
       }
 
@@ -670,7 +669,6 @@ namespace __variant
              this->_M_reset();
            return {};
          }, __variant_cast<_Types...>(__rhs));
-       __glibcxx_assert(this->_M_index == __rhs._M_index);
        return *this;
       }
 
@@ -1245,7 +1243,7 @@ namespace __variant
        noexcept(is_nothrow_constructible_v<__accepted_type<_Tp&&>, _Tp&&>)
        : variant(in_place_index<__accepted_index<_Tp&&>>,
                  std::forward<_Tp>(__t))
-       { __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this)); }
+       { }
 
       template<typename _Tp, typename... _Args,
               typename = enable_if_t<__exactly_once<_Tp>
@@ -1254,7 +1252,7 @@ namespace __variant
        variant(in_place_type_t<_Tp>, _Args&&... __args)
        : variant(in_place_index<__index_of<_Tp>>,
                  std::forward<_Args>(__args)...)
-       { __glibcxx_assert(holds_alternative<_Tp>(*this)); }
+       { }
 
       template<typename _Tp, typename _Up, typename... _Args,
               typename = enable_if_t<__exactly_once<_Tp>
@@ -1265,7 +1263,7 @@ namespace __variant
                _Args&&... __args)
        : variant(in_place_index<__index_of<_Tp>>, __il,
                  std::forward<_Args>(__args)...)
-       { __glibcxx_assert(holds_alternative<_Tp>(*this)); }
+       { }
 
       template<size_t _Np, typename... _Args,
               typename = enable_if_t<
@@ -1274,7 +1272,7 @@ namespace __variant
        variant(in_place_index_t<_Np>, _Args&&... __args)
        : _Base(in_place_index<_Np>, std::forward<_Args>(__args)...),
        _Default_ctor_enabler(_Enable_default_constructor_tag{})
-       { __glibcxx_assert(index() == _Np); }
+       { }
 
       template<size_t _Np, typename _Up, typename... _Args,
               typename = enable_if_t<is_constructible_v<__to_type<_Np>,
@@ -1284,7 +1282,7 @@ namespace __variant
                _Args&&... __args)
        : _Base(in_place_index<_Np>, __il, std::forward<_Args>(__args)...),
        _Default_ctor_enabler(_Enable_default_constructor_tag{})
-       { __glibcxx_assert(index() == _Np); }
+       { }
 
       template<typename _Tp>
        enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
@@ -1300,7 +1298,6 @@ namespace __variant
            std::get<__index>(*this) = std::forward<_Tp>(__rhs);
          else
            this->emplace<__index>(std::forward<_Tp>(__rhs));
-         __glibcxx_assert(holds_alternative<__accepted_type<_Tp&&>>(*this));
          return *this;
        }
 
@@ -1309,10 +1306,8 @@ namespace __variant
                    _Tp&>
        emplace(_Args&&... __args)
        {
-         auto& ret =
-           this->emplace<__index_of<_Tp>>(std::forward<_Args>(__args)...);
-         __glibcxx_assert(holds_alternative<_Tp>(*this));
-         return ret;
+         constexpr size_t __index = __index_of<_Tp>;
+         return this->emplace<__index>(std::forward<_Args>(__args)...);
        }
 
       template<typename _Tp, typename _Up, typename... _Args>
@@ -1321,11 +1316,8 @@ namespace __variant
                    _Tp&>
        emplace(initializer_list<_Up> __il, _Args&&... __args)
        {
-         auto& ret =
-           this->emplace<__index_of<_Tp>>(__il,
-                                          std::forward<_Args>(__args)...);
-         __glibcxx_assert(holds_alternative<_Tp>(*this));
-         return ret;
+         constexpr size_t __index = __index_of<_Tp>;
+         return this->emplace<__index>(__il, std::forward<_Args>(__args)...);
        }
 
       template<size_t _Np, typename... _Args>
@@ -1363,7 +1355,6 @@ namespace __variant
              this->_M_index = variant_npos;
              __throw_exception_again;
            }
-         __glibcxx_assert(index() == _Np);
          return std::get<_Np>(*this);
        }
 
@@ -1399,7 +1390,6 @@ namespace __variant
              this->_M_index = variant_npos;
              __throw_exception_again;
            }
-         __glibcxx_assert(index() == _Np);
          return std::get<_Np>(*this);
        }