[Bug c++/51547] New: auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[Bug c++/51547] New: auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51547

             Bug #: 51547
           Summary: auto, type deduction, reference collapsing and const:
                    invalid initialization of reference of type 'const
                    X&&' from expression of type 'const X'
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tudorb@fb.com


Created attachment 26074
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26074
g++ -E output

The following program fails to compile using g++ 4.6.2 (g++ -std=c++0x)

#include <vector>

struct Bar {
  int x;
};

struct Foo {
  const std::vector<Bar>& bar() const {
    return bar_;
  }

  std::vector<Bar> bar_;
};

template <class X>
struct Y {
  void foo() {
    Foo a;
    auto b = a.bar().begin();
    auto&& c = b->x;
  }
};

template <class X>
void foo() {
  Foo a;
  auto b = a.bar().begin();
  auto&& c = b->x;
}


int main() {
  Y<int> p;
  p.foo();
  foo<int>();
}

a.cpp: In member function ‘void Y<X>::foo() [with X = int]’:
a.cpp:34:9:   instantiated from here
a.cpp:20:19: error: invalid initialization of reference of type ‘const int&&’
from expression of type ‘const int’
a.cpp: In function ‘void foo() [with X = int]’:
a.cpp:35:12:   instantiated from here
a.cpp:28:17: error: invalid initialization of reference of type ‘const int&&’
from expression of type ‘const int’

The program compiles correctly if Y or foo() are non-templates.

Reference collapsing should deduce "auto&& c" as "const int&", and it appears
to do so in the non-template case.

Note that this bug is very brittle to reproduce (I tried to reduce it to remove
the use of std::vector, but I couldn't) -- for example, replacing "b->x" with
"(*b).x" makes the code compile correctly.

Attaching the preprocessor output (g++ -std=c++0x -E)

[Bug c++/51547] auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51547

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
                 CC|                            |jason at gcc dot gnu.org
      Known to work|                            |4.7.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-12-14 11:34:39 UTC ---
compiles fine with 4.7.0 20111126, but I'm not sure if it's fixed or just
latent

[Bug c++/51547] auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[paolo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51547

--- Comment #2 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2011-12-27 19:04:28 UTC ---
Author: paolo
Date: Tue Dec 27 19:04:24 2011
New Revision: 182695

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=182695
Log:
2011-12-27  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/51547
    * g++.dg/cpp0x/pr51547.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/pr51547.C

[Bug c++/51547] auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51547

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.7.0

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-27 19:07:23 UTC ---
Testcase added, I'm closing the PR as fixed for 4.7.0.

[Bug c++/51547] auto, type deduction, reference collapsing and const: invalid initialization of reference of type 'const X&&' from expression of type 'const X'

[paolo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51547

--- Comment #4 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2011-12-28 15:53:01 UTC ---
Author: paolo
Date: Wed Dec 28 15:52:54 2011
New Revision: 182709

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=182709
Log:
2011-12-27  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/51547
    * g++.dg/cpp0x/pr51547.C: New.

Modified:
    trunk/gcc/testsuite/ChangeLog