[Bug c++/52748] New: [C++11] N3276 changes to decltype

[Bug c++/52748] New: [C++11] N3276 changes to decltype

[zeratul976 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

             Bug #: 52748
           Summary: [C++11] N3276 changes to decltype
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zeratul976@hotmail.com


The C++11 standard section 5.2.2 pararaph 11 states that:

"If a function call is a prvalue of object type [and] if the function call is
[...] the operand of a decltype-specifier [...], a temporary object is not
introduced for the prvalue. The type of the prvalue may be incomplete. [Note:
as a result, storage is not allocated for the prvalue and it is not destroyed;
thus, a class type is not instantiated as a result of being the type of a
function call in this context." 

This addition was made late in the C++11 standardization process; its rationale
is described in the paper N3276 [1]

It seems GCC does not currently support this. In fact, when run on the "trivial
example" given in N3276, GCC gets into an infinite loop and, if left running,
uses up all the memory in the system (!) (Should I file that as a separate
issue?)

Here is the example (WARNING - may exhaust your system's virtual memory if
you're not careful):

template<class T> struct S;
template<class X, class Y> struct pair {};
template<class T> S<T> wrap(T) { return 0; }
template<class T>
struct S
{
   S(int = 0) {}
   decltype(wrap(pair<T,T>())) foo() { return 0; } // ERROR
   S<pair<T,T> > bar() { return 0; } // OK
};
S<int> s;


[1] http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2011/n3276.pdf

[Bug c++/52748] [C++11] N3276 changes to decltype

[lundberj at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #1 from Johan Lundberg <lundberj at gmail dot com> 2012-08-09 08:08:46 UTC ---
confirmed... I agree with this. Among other things it's important for
boost/C++11 interop.

[Bug c++/52748] [C++11] N3276 changes to decltype

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-08-10
     Ever Confirmed|0                           |1

[Bug c++/52748] [C++11] N3276 changes to decltype

[zeratul976 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #2 from Nathan Ridge <zeratul976 at hotmail dot com> 2012-11-07 04:31:24 UTC ---
Clang deemed this issue important enough to warrant a new entry ("Incomplete
retrn types", under "Declared type of an expression") in their C++11 status
page.

[Bug c++/52748] [C++11] N3276 changes to decltype

[zeratul976 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #3 from Nathan Ridge <zeratul976 at hotmail dot com> 2012-11-07 04:32:06 UTC ---
(In reply to comment #2)
> Clang deemed this issue important enough to warrant a new entry ("Incomplete
> retrn types", under "Declared type of an expression") in their C++11 status
> page.

Link: http://clang.llvm.org/cxx_status.html

[Bug c++/52748] [C++11] N3276 changes to decltype

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2013-02-18 12:47:32 UTC ---
*** Bug 56374 has been marked as a duplicate of this bug. ***

[Bug c++/52748] [C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |jason at gcc dot gnu.org
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |

[Bug c++/52748] [C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-17 02:37:21 UTC ---
Author: jason
Date: Sun Mar 17 02:37:09 2013
New Revision: 196736

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=196736
Log:
    N3276
    PR c++/52748
    * cp-tree.h (tsubst_flags): Add tf_decltype.
    * call.c (build_cxx_call): Don't build a temporary if it's set.
    (build_over_call): Make sure it's only passed to build_cxx_call.
    * parser.c (cp_parser_primary_expression): Add decltype_p parm.
    (cp_parser_unary_expression): Likewise.
    (cp_parser_cast_expression): Likewise.
    (cp_parser_binary_expression): Likewise.
    (cp_parser_assignment_expression): Likewise.
    (cp_parser_postfix_expression): Likewise.  Pass tf_decltype.
    (cp_parser_explicit_instantiation): Add decltype_p.  Force a
    temporary for a call on the LHS of a comma.
    (cp_parser_decltype): Pass true to decltype_p parms.
    * pt.c (tsubst) [DECLTYPE_TYPE]: Pass tf_decltype.
    (tsubst_copy_and_build): Pass tf_decltype down only for
    CALL_EXPR and the RHS of COMPOUND_EXPR.
    * tree.c (build_cplus_new): Call complete_type_or_maybe_complain.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/decltype-call1.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/cp/cp-tree.h
    trunk/gcc/cp/parser.c
    trunk/gcc/cp/pt.c
    trunk/gcc/cp/tree.c

[Bug c++/52748] [C++11] N3276 changes to decltype

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

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

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-03-23 10:52:25 UTC ---
Fixed 4.8.1 too.

[Bug c++/52748] [C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.8.1

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-26 14:31:48 UTC ---
Closing.

[Bug c++/52748] [C++11] N3276 changes to decltype

[mimomorin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

Michel Morin <mimomorin at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mimomorin at gmail dot com

--- Comment #8 from Michel Morin <mimomorin at gmail dot com> 2013-03-28 13:11:00 UTC ---
The implementation of N3276 is not complete. 

`decltype` in gcc-4.9-20130324 does not require type-completeness 
but involves unnecessary template instantiations: 

* Compilation of the demonstration code in N3276 (also appeared in 
Nathan Ridge's comment in this PR) exhausted memory on my PC, 
so I had to quit the compilation.

* Boost's testing code
(https://svn.boost.org/svn/boost/trunk/libs/config/test/boost_no_decltype_n3276.ipp)
failed to compile. 
(One needs to remove `namespace boost_no_cxx11_decltype_n3276 {` and its
closing 
brace`}`, and rename `int test()` to `int main()` before the compilation.)

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |
            Summary|[C++11] N3276 changes to    |[4.9 Regression][C++11]
                   |decltype                    |N3276 changes to decltype

--- Comment #9 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-03-28 13:29:00 UTC ---
Jason is the right person to answer your first and third question, but the
answer to the second one is simple: we have a regression in mainline, current
4_8-branch works fine.

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-28 13:36:07 UTC ---
The implementation is complete.  Unfortunately, my fixes for DRs 337 and 657
are interfering with the desired result; creating a function type returning an
abstract class causes deduction to fail, so we instantiate the return type of
wrap in order to determine whether it is abstract.  I guess I'll disable that
instantiation and raise this issue with Core...

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #12 from Jason Merrill <jason at gcc dot gnu.org> 2013-03-28 20:02:57 UTC ---
Fixed again.  I added a different testcase whose failure mode is an error,
rather than consuming all resources.

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[mimomorin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

--- Comment #13 from Michel Morin <mimomorin at gmail dot com> 2013-03-29 00:40:59 UTC ---
Thanks Jason, Paolo. 
I'll enable N3276 decltype support in Boost.Config for gcc 4.8.1 and 4.9.0.

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[zeratul976 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

Nathan Ridge <zeratul976 at hotmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |

--- Comment #14 from Nathan Ridge <zeratul976 at hotmail dot com> 2013-04-10 09:27:10 UTC ---
Here is a related example that still fails to compile:


template <int> struct A;

template <typename T> struct B : A<sizeof(T)> {};

template <typename T>
B<typename T::type> operator-(T);

struct S
{
    struct F
    {
        typedef S type;
    };

    auto foo() -> decltype(-F());
};



This is compiled successfully by clang, but GCC (4.9, r197663) gives the
following error:

test.cpp: In instantiation of 'struct B<S>':
test.cpp:15:31:   required from here
test.cpp:3:42: error: invalid application of 'sizeof' to incomplete type 'S'
 template <typename T> struct B : A<sizeof(T)> {};
                                          ^


If the operator- is replaced by a non-operator function, GCC compiles the
example successfully, which leads me to believe it's a bug.

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #15 from Jason Merrill <jason at gcc dot gnu.org> 2013-04-11 21:46:33 UTC ---
(In reply to comment #14)
> Here is a related example that still fails to compile:

Fixed.

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[zeratul976 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

Nathan Ridge <zeratul976 at hotmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |

--- Comment #17 from Nathan Ridge <zeratul976 at hotmail dot com> 2013-04-12 07:23:07 UTC ---
(Sorry, meant to reopen with last comment.)

[Bug c++/52748] [4.9 Regression][C++11] N3276 changes to decltype

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52748

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #18 from Jason Merrill <jason at gcc dot gnu.org> 2013-04-16 21:24:12 UTC ---
Fixed.