[Bug c++/63723] New: Narrowing conversion allowed in braced init list in SFINAE context

[Bug c++/63723] New: Narrowing conversion allowed in braced init list in SFINAE context

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

            Bug ID: 63723
           Summary: Narrowing conversion allowed in braced init list in
                    SFINAE context
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tavianator at gmail dot com

Created attachment 33877
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33877&action=edit
Preprocessed source

GCC allows narrowing conversions in braced init lists with only a warning.  The
documentation suggests that this extension doesn't apply in SFINAE contexts,
but it appears to be wrong:

$ cat foo.cpp
#include <type_traits>
#include <utility>

template <typename From, typename To>
class is_list_convertible_helper
{
  template <typename To2>
  static void requires_conversion(To2 t);

  template <typename From2, typename To2,
      typename = decltype(requires_conversion<To2>({std::declval<From2>()}))>
  //                                               ^ Braced initializer
  static std::true_type helper(int);

  template <typename From2, typename To2>
  static std::false_type helper(...);

public:
  using type = decltype(helper<From, To>(0));
};

template <typename From, typename To>
class is_list_convertible
  : public is_list_convertible_helper<From, To>::type
{ };

static_assert(!is_list_convertible<double, int>::value,
    "double -> int is narrowing!");

$ g++ -std=c++11 foo.cpp
foo.cpp: In substitution of ‘template<class From2, class To2, class> static
std::true_type is_list_convertible_helper<From, To>::helper(int) [with From2 =
double; To2 = int; <template-parameter-1-3> = <missing>]’:
foo.cpp:18:31:   required from ‘class is_list_convertible_helper<double, int>’
foo.cpp:22:7:   required from ‘class is_list_convertible<double, int>’
foo.cpp:26:48:   required from here
foo.cpp:10:46: warning: narrowing conversion of ‘std::declval<double>()’ from
‘double’ to ‘int’ inside { } [-Wnarrowing]
       typename = decltype(requires_conversion<To2>({std::declval<From2>()}))>
                                              ^
foo.cpp:26:1: error: static assertion failed: double -> int is narrowing!
 static_assert(!is_list_convertible<double, int>::value,
 ^
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From: "dave.anglin at bell dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/63691] GCC 4.9.x fails to build GLIBC 2.20 on HPPA
Date: Mon, 03 Nov 2014 15:30:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc691

--- Comment #12 from dave.anglin at bell dot net ---
On 10/31/2014 5:30 PM, aaro.koskinen at iki dot fi wrote:
> I was able to isolate the problem to GLIBC elf/dl-conflict.c and specifically
> hppa specific elf_machine_rela function called from there.
Could you isolate the "wrong code" in elf/dl-conflict.c?  It seems a null
pointer check in this object is being incorrectly removed.

Dave

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
and what is that third template parameter? 

‘template<class From2, class To2, class> static std::true_type
is_list_convertible_helper<From, To>::helper(int) [with From2 = double; To2 =
int; <template-parameter-1-3> = <missing>]’
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc721

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |5.0

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

--- Comment #2 from Tavian Barnes <tavianator at gmail dot com> ---
It's decltype(requires_conversion<To2>({std::declval<From2>()})).  Not sure why
it says <missing>.

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler@googlemail.
                   |                            |com

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
The problem seems to be fixed in gcc HEAD 5.0.0 20141103 (experimental).
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Subject: [Bug java/41991] gcj segfaults on i686-apple-darwin9 and x86_64-apple-darwin9
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idA991
Bug 41991 depends on bug 44107, which changed state.

Bug 44107 Summary: gcc emits frame (epilogue) info incompatible with the darwin {8,9}-unwinder,10-compacter
https://gcc.gnu.org/bugzilla/show_bug.cgi?idD107

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

--- Comment #4 from Tavian Barnes <tavianator at gmail dot com> ---
Still happens with 4.9.2 though.  Is a backport of the fix possible?

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-12-18
     Ever confirmed|0                           |1

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This isn't a regression, thus for the time being I'm going to simply add a
testcase. If you want to argue for a backport, please figure out the commit
which fixed the bug and send a message to gcc-patches.

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

--- Comment #6 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Fri Dec 19 00:02:05 2014
New Revision: 218880

URL: https://gcc.gnu.org/viewcvs?rev=218880&root=gcc&view=rev
Log:
2014-12-18  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/63723
    * g++.dg/cpp0x/sfinae54.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/sfinae54.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0

--- Comment #7 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.

[Bug c++/63723] Narrowing conversion allowed in braced init list in SFINAE context

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63723

--- Comment #8 from Tavian Barnes <tavianator at gmail dot com> ---
Actually it is a regression: according to http://gcc.godbolt.org/, GCC 4.6.4
compiled it correctly with -std=c++0x and

  typedef decltype(helper<From, To>(0)) type;

instead of

  using type = decltype(helper<From, To>(0));

while still allowing

  int x{1.0};