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4458. indirect<T> should be convertible to T&

Section: 20.4.1.6 [indirect.obs] Status: New Submitter: Zhihao Yuan Opened: 2025-11-05 Last modified: 2025-11-06

Priority: Not Prioritized

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Discussion:

In addition to serving as a better unique_ptr<T> in Pimpl, indirect<T> can also appear as a drop-in replacement for T when T may conditionally be an incomplete type. In which case, if indirect<T> is convertible to T&, certain tasks can be done without meta-programming (e.g., returning the object of type T or indirect<T> from a function with a return type T&).

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.4.1.2 [indirect.syn], as indicated:

   namespace std {
     template<class T, class Allocator = allocator<T>>
     class indirect {
     public:
       […]
       // 20.4.1.6 [indirect.obs], observers
       constexpr operator const T& () const & noexcept;
       constexpr operator T& () & noexcept;
       constexpr operator const T&& () const && noexcept;
       constexpr operator T&& () && noexcept;
       constexpr const T& operator*() const & noexcept;
       constexpr T& operator*() & noexcept;
       constexpr const T&& operator*() const && noexcept;
       constexpr T&& operator*() && noexcept;
       constexpr const_pointer operator->() const noexcept;
       constexpr pointer operator->() noexcept;
       […]
   

2. Modify 20.4.1.6 [indirect.obs], as indicated:

   constexpr operator const T& () const & noexcept;
   constexpr operator T& () & noexcept;
   constexpr const T& operator*() const & noexcept;
   constexpr T& operator*() & noexcept;
   

   -1- Preconditions: *this is not valueless.

   -2- Returns: *p.

   constexpr operator const T&& () const && noexcept;
   constexpr operator T&& () && noexcept;
   constexpr const T&& operator*() const && noexcept;
   constexpr T&& operator*() && noexcept;
   

   -3- Preconditions: *this is not valueless.

   -4- Returns: std::move(*p).