Issue 2993: reference_wrapper<T> conversion from T&&

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2993. `reference_wrapper<T>` conversion from `T&&`

Section: 22.10.6 [refwrap] Status: C++20 Submitter: Tim Song Opened: 2017-06-28 Last modified: 2021-02-25

Priority: 3

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Discussion:

reference_wrapper<T> has a deleted constructor taking T&& in order to prevent accidentally wrapping an rvalue (which can otherwise happen with the reference_wrapper(T&) constructor if T is a non-volatile const-qualified type). Unfortunately, a deleted constructor can still be used to form implicit conversion sequences, so the deleted T&& constructor has the (presumably unintended) effect of creating an implicit conversion sequence from a T rvalue to a reference_wrapper<T>, even though such a conversion would be ill-formed if actually used. This is visible in overload resolution:

void meow(std::reference_wrapper<int>); //#1
void meow(convertible_from_int); //#2
meow(0); // error, ambiguous; would unambiguously call #2 if #1 instead took int&

and in conditional expressions (and hence std::common_type) after core issue 1895:

std::reference_wrapper<int> purr();

auto x = true? purr() : 0; // error, ambiguous: ICS exists from int prvalue to 
                           // reference_wrapper<int> and from reference_wrapper<int> to int

using t = std::common_type_t<std::reference_wrapper<int>, int>; // error: no member 'type' because the conditional 
                                                                // expression is ill-formed

The latter in turn interferes with the use of reference_wrapper as a proxy reference type with proxy iterators.

We should ensure that there is no implicit conversion sequence from T rvalues to reference_wrapper<T>, not just that the conversion will be ill-formed when used. This can be done by using a suitably constrained constructor template taking a forwarding reference instead of the current pair of constructors taking T& and T&&.

[2017-06-29, Tim adds P/R and comments]

The draft P/R below uses a conditional noexcept specification to ensure that converting a T& to a reference_wrapper<T> remains noexcept and make it not usable when the source type is a reference_wrapper of the same type so as to avoid affecting is_trivially_constructible. It adds a deduction guide as the new constructor template will not support class template argument deduction.

The constructor template has the additional effect of making reference_wrapper<T> convertible from everything that is convertible to T&. This implies, for instance, that reference_wrapper<int> is now convertible to reference_wrapper<const int> when it wasn't before (the conversion would have required two user-defined conversions previously). This more closely emulates the behavior of an actual reference, but does represent a change to the existing behavior.

If perfectly emulating the existing behavior is desired, a conditionally-explicit constructor that is only implicit if T is reference-compatible with remove_reference_t (see 9.4.4 [dcl.init.ref]) can be used.

[2017-07 Toronto Tuesday PM issue prioritization]

Priority 3; what else in the library does this affect? ref or cref?

[2016-07, Toronto Saturday afternoon issues processing]

Status to Ready.

Proposed resolution:

This wording is relative to N4659.

Edit 22.10.6 [refwrap], class template reference_wrapper synopsis, as indicated:

namespace std {
  template <class T> class reference_wrapper {
    […]
    // construct/copy/destroy
    reference_wrapper(T&) noexcept;
    reference_wrapper(T&&) = delete;    // do not bind to temporary objects
    template <class U>
      reference_wrapper(U&&) noexcept(see below);
    […]
  };
  template <class T>
  reference_wrapper(T&) -> reference_wrapper<T>;
  […]
}

Edit 22.10.6.2 [refwrap.const]/1 as indicated:
```
reference_wrapper(T& t) noexcept;
```
~~-1- Effects: Constructs a reference_wrapper object that stores a reference to t.~~
```
template<class U>
 reference_wrapper(U&& u) noexcept(see below);
```
-?- Remarks: Let FUN denote the exposition-only functions
```
void FUN(T&) noexcept;
void FUN(T&&) = delete;
```
This constructor shall not participate in overload resolution unless the expression FUN(declval()) is well-formed and is_same_v<decay_t, reference_wrapper> is false. The expression inside noexcept is equivalent to noexcept(FUN(declval())).
-?- Effects: Creates a variable r as if by T& r = std::forward(u), then constructs a reference_wrapper object that stores a reference to r.

2993. reference_wrapper<T> conversion from T&&

2993. `reference_wrapper<T>` conversion from `T&&`