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2534. Constrain rvalue stream operators

Section: 31.7.6.6 [ostream.rvalue], 31.7.5.6 [istream.rvalue] Status: C++17 Submitter: Robert Haberlach Opened: 2015-09-08 Last modified: 2017-07-30

Priority: 3

View all other issues in [ostream.rvalue].

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Discussion:

The rvalue stream insertion and extraction operators should be constrained to not participate in overload resolution unless the expression they evaluate is well-formed. Programming code that tests the validity of stream insertions (or extractions) using SFINAE can result in false positives, as the present declarations accept virtually any right-hand side argument. Moreover, there is no need for pollution of the candidate set with ill-formed specializations.

[2016-08 - Chicago]

Thurs AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4527.

1. Modify 31.7.6.6 [ostream.rvalue] as indicated:

   template <class charT, class traits, class T>
     basic_ostream<charT, traits>&
     operator<<(basic_ostream<charT, traits>&& os, const T& x);
   

   -1- Effects: os << x

   -2- Returns: os

   -?- Remarks: This function shall not participate in overload resolution unless the expression os << x is well-formed.

2. Modify 31.7.5.6 [istream.rvalue] as indicated:

   template <class charT, class traits, class T>
     basic_istream<charT, traits>&
     operator>>(basic_istream<charT, traits>&& is, T& x);
   

   -1- Effects: is >> x

   -2- Returns: is

   -?- Remarks: This function shall not participate in overload resolution unless the expression is >> x is well-formed.