[Bug libstdc++/52114] New: SFINAE out the rvalue iostream operators to give better error messages

[b.r.longbons at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52114

             Bug #: 52114
           Summary: SFINAE out the rvalue iostream operators to give
                    better error messages
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: b.r.longb...@gmail.com


Created attachment 26566
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26566
Use SFINAE in delayed return type to make sure there's an lvalue version

Currently, if there is no stream insertion/extraction operator defined for a
class, it tries to use the template that takes an rvalue istream or ostream and
anything on the right.

Actual Result:
error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to
‘std::basic_ostream<char>&&’
/usr/include/c++/4.6/ostream:581:5: error:   initializing argument 1 of
‘std::basic_ostream<_CharT, _Traits>&
std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT
= char, _Traits = std::char_traits<char>, _Tp = Foo]’

Expected Result:
error: no match for ‘operator<<’ in ‘std::cout << Foo()’
note: candidates are:

Note that this patch is not safe to apply to 4.6 because of bug 51878