Proposed resolution:

This wording is relative to N4606.

1. Change return type of emplace in class definition in [queue.defn]:

   template <class... Args>
     referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
   

2. Change return type of emplace in class definition in [stack.defn]:

   template <class... Args>
     referencedecltype(auto) emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }
   

[ 2017-03-04, Kona ]

Status to Tentatively Ready.

[ 2016-11-12, Issaquah ]

Sat AM: P2

The stack and queue adaptors are now defined as:

template <class... Args>
reference emplace(Args&&... args) { return c.emplace_back(std::forward<Args>(args)...); }

This breaks any code using queue<UserDefinedSequence> or stack<UserDefinedSequence> until the user-defined containers are updated to meet the new C++17 requirements.

If we defined them as returning decltype(auto) then we don't break any code. When used with std::vector or std::deque they will return reference, as required, but when used with C++14-conforming containers they will return void, as before.