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Subject: [Bug c++/51829] New: decltype() deduces non-const but only in a template
Date: Wed, 11 Jan 2012 21:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51829

             Bug #: 51829
           Summary: decltype() deduces non-const but only in a template
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bugs@sehe.nl


The problem rejects a valid `auto expr = ...;` initialization. I have reduced
things to the following (full preprocessed version attached)

The template arguments aren't relevant (unused).

    #include <boost/spirit/include/qi_eps.hpp>

    template <typename> // without template, works!
    void bug()
    {
        const auto& e1 = boost::spirit::qi::eps(false);
        const auto& e2 = e1 | '1';

        decltype(boost::spirit::qi::eps(false) | '1') *notok;

        notok = &e2; // FAILS if in template function
    }

    int main() { bug<void>(); }

By using a pointer declaration for `notok` I was able to use demangling to
print the difference between the deduced types:

    boost::proto::exprns_::expr<boost::proto::tagns_::tag::bitwise_or,
boost::proto::argsns_::list2<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,
boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::eps,
boost::fusion::vector1<bool> > >, 0l> const&,
boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,
boost::proto::argsns_::term<char const&>, 0l> >, 2l>
    boost::proto::exprns_::expr<boost::proto::tagns_::tag::bitwise_or,
   
boost::proto::argsns_::list2<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,
   
boost::proto::argsns_::term<boost::spirit::terminal_ex<boost::spirit::tag::eps,
    boost::fusion::vector1<bool> > >, 0l> CONST&,
    boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,
    boost::proto::argsns_::term<char const&>, 0l> >, 2l> const*

The 'CONST' I capitalized in the middle, is missing when the decltype() is in a
function template, which prevents the assignment.

I don't know how to further reduce the testcase at this point in time. MSVC
2010 compiles the code without problem.