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3046. Do not require reference_wrapper to support non-referenceable function types

Section: 20.14.6 [refwrap] Status: New Submitter: Jonathan Wakely Opened: 2017-12-14 Last modified: 2020-09-06

Priority: 3

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[refwrap] says that reference_wrapper<T> is a "wrapper around a reference to an object or function of type T" but this doesn't actually constrain it, and doesn't forbid non-referenceable function types like int() const.

There is no way to construct a reference_wrapper<int() const> but implementations are required to provide partial specializations for functions with cv-qualifiers and ref-qualifiers in order to define a nested result_type. It should be undefined to instantiate reference_wrapper<T> with a non-referenceable type, or with a reference type (since references to references are not possible). Making it undefined (rather than ill-formed or unspecified) means implementations are not required to diagnose such invalid specializations, but also don't have to go to the effort of supporting weak result types etc.

[2018-01; Priority set to 3 after mailing list discussion]

Previous resolution [SUPERSEDED]:

This wording is relative to N4713.

  1. Modify 20.14.6 [refwrap] as indicated:

    -1- reference_wrapper<T> is a CopyConstructible and CopyAssignable wrapper around a reference to an object or function of type T. T shall be a referenceable type (3.45 [defns.referenceable]) that is not a reference type.

    -2- reference_wrapper<T> shall be a trivially copyable type (6.8 [basic.types]).

[2019-03-15; Daniel comments and provides revised wording]

The current wording is now far behind the working draft and a synchronization is therefore recommended. In particular, with the acceptance of P0357R1, the specification of reference_wrapper has no longer any weak result type. Second, I would like to concur with a remark from Tomasz to change the wording to replace the undefined behavior by an ill-formed program instead, because every attempt to instantiate the definition of reference_wrapper will instantiate its member declarations, and this would cause the program to become ill-formed anyway because of the illegal formation of references to non-referenceable function types for member functions such as T& get() const noexcept.

As concrete wording suggestion I would recommend wording that ensures that an ill-formed program is only required when a specialization of reference_wrapper is instantiated, because in the absence of a constrained template parameter we shouldn't require implementations to diagnose even forming the name of a reference_wrapper specialization such as in the following example:

using X = reference_wrapper<int() const>;

The wording below does not take advantage of a Mandates: element to prevent a dependency on LWG 3193 and because such an element is rarely used to specify class templates. If the committee wishes to use such an element, the equivalent wording would be:

Mandates: reference_wrapper is instantiated with a referenceable type (3.45 [defns.referenceable]) as the argument for the template parameter T.

Proposed resolution:

This wording is relative to N4800.

  1. Modify 20.14.6 [refwrap] as indicated:

    -1- reference_wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T. If reference_wrapper is instantiated with a non-referenceable type (3.45 [defns.referenceable]) as the argument for the template parameter T, the program is ill-formed.

    -2- reference_wrapper<T> is a trivially copyable type (6.8 [basic.types]).

    -3- The template parameter T of reference_wrapper may be an incomplete type.