[Bug c++/47999] New: [C++0x] auto type deduction works incorrectly in a function template

[Bug c++/47999] New: [C++0x] auto type deduction works incorrectly in a function template

[mimomorin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

           Summary: [C++0x] auto type deduction works incorrectly in a
                    function template
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mimomorin@gmail.com


Created attachment 23554
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23554
Test case

When `auto&&` is initialized with a function return value that is an lvalue
reference, the deduced type should be an lvalue reference. 

    int& identity(int& i) { return i; }

    void f()
    {
        int i = 0;
        auto&& x = identity(i); // x has type `int&`
    }

But, in a function template, auto type deduction incorrectly deduces
the type as an rvalue reference. 

    template <typename T>
    void f()
    {
        int i = 0;
        auto&& x = identity(i); // BUG: x has type `int&&`
    }

This bug leads to compilation error in gcc 4.5 and 4.6, 
because lvalues cannot bind to rvalue references. 
In gcc 4.4, the code compiles fine because lvalues are 
allowed to bind to rvalue references.

Tested compilers: gcc-4.4.5, 4.5.2, 4.6-20110219.
(This problem was found by Nathan Ridge.)

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-03-05 23:39:18 UTC ---
Let's CC Jason,

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011.03.08 13:47:09
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-08 15:14:02 UTC ---
Created attachment 23583
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23583
Fix

Here's a patch that fixes the bug and passes regression testing.  I don't plan
to put it in for 4.6.0 as I'm concerned about possible side-effects; I seem to
remember trying to do exactly this once before and it causing problems.  I'll
put it in for 4.7 once 4.6 branches, and then perhaps into 4.6.1.

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-16 17:04:35 UTC ---
Author: jason
Date: Wed Mar 16 17:04:30 2011
New Revision: 171053

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=171053
Log:
    PR c++/47999
    * semantics.c (finish_call_expr): Preserve reference semantics
    in templates.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/auto22.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-29 14:25:49 UTC ---
Author: jason
Date: Tue Mar 29 14:25:37 2011
New Revision: 171670

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=171670
Log:
    PR c++/47999
    * semantics.c (finish_call_expr): Preserve reference semantics
    in templates.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/auto22.C
Modified:
    branches/gcc-4_6-branch/gcc/cp/ChangeLog
    branches/gcc-4_6-branch/gcc/cp/semantics.c
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog

[Bug c++/47999] [C++0x] auto type deduction works incorrectly in a function template

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47999

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.1

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-29 14:38:30 UTC ---
Fixed for 4.6.1.