[Bug c++/50248] New: gcc confused, tries to use variadic template to copy itself when it should use default constructor

["b.r.longbons at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50248

             Bug #: 50248
           Summary: gcc confused, tries to use variadic template to copy
                    itself when it should use default constructor
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: b.r.longbons@gmail.com


Created attachment 25149
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25149
far smaller example than the 30000 lines where I first encountered the bug

GCC is trying to instantiate the variadic template as if to copy itself, but
the only thing it should be doing is calling the default constructor.

It only happens when it is constexpr and uses rvalue references.

It only happens when it the parent class has a deleted copy constructor that
takes a *nonconst* reference.

4.5.3 compiles it, but that's because it actually ignores the constexpr
keyword.
Known bad: 4.6.0, 4.6.1-4 debian, svn trunk as of 20110804

Workarounds:
1. Don't use rvalue references.
2. Add a fixed parameter before the variadic one.

Error message:

bug.cpp: In constructor 'constexpr earray<Elt, max>::earray(Elt2&& ...) [with
Elt2 = {earray<short int, 11u>&}, Elt = short int, unsigned int max = 11u]':
bug.cpp:25:30:   instantiated from here
bug.cpp:9:50: error: cannot convert 'earray<short int, 11u>' to 'short int' in
initialization
bug.cpp:9: confused by earlier errors, bailing out