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3423. std::any_cast should never return a cv-qualified type

Section: 22.7.5 [any.nonmembers] Status: New Submitter: Casey Carter Opened: 2020-04-02 Last modified: 2020-09-06

Priority: 3

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Discussion:

The return type of the non-pointer overloads of std::any_cast<T> is T. This is silly when T is cv-qualified (and, since merging P1152 "Deprecating volatile" into the working draft, deprecated when T is volatile-qualified). We should strip cv-qualifiers to determine the type returned.

[2020-04-18 Issue Prioritization]

Priority to 3 after reflector discussion.

Proposed resolution:

This wording is relative to N4861.

1. Modify 22.7.2 [any.synop] as indicated:

   […]
   template<class T>
     remove_cv_t<T> any_cast(const any& operand);
   template<class T>
     remove_cv_t<T> any_cast(any& operand);
   template<class T>
     remove_cv_t<T> any_cast(any&& operand);
   […]
   

2. Modify 22.7.5 [any.nonmembers] as indicated:

   template<class T>
     remove_cv_t<T> any_cast(const any& operand);
   template<class T>
     remove_cv_t<T> any_cast(any& operand);
   template<class T>
     remove_cv_t<T> any_cast(any&& operand);
   

   -4- Let R be the type remove_cv_t<T>, and U be the type remove_cvref_t<T>.

   -5- Mandates: For the first overload, is_constructible_v<TR, const U&> is true. For the second overload, is_constructible_v<TR, U&> is true. For the third overload, is_constructible_v<TR, U> is true.

   -6- Returns: For the first and second overload, static_cast<TR>(*any_cast<U>(&operand)). For the third overload, static_cast<TR>(std::move(*any_cast<U>(&operand))).

   -7- Throws: bad_any_cast if operand.type() != typeid(remove_reference_t<T>).