[Bug c++/57820] New: Can't initialize a constexpr object with inline initialization

[Bug c++/57820] New: Can't initialize a constexpr object with inline initialization

[liweifriends at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

            Bug ID: 57820
           Summary: Can't initialize a constexpr object with inline
                    initialization
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: liweifriends at gmail dot com

the source code:

struct C
{
    int a = 2;
    int b = a + 1;
};

C c;  //OK
constexpr C d; //error

int main()
{
}

Why I can't initialize d?

[Bug c++/57820] Can't initialize a constexpr object with inline initialization

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-07-04
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it's invalid unless you provide an initializer or the class has a
default constructor e.g.

constexpr C d = {};

G++ still rejects that though, with a very unhelpful diagnostic:

t.cc:8:18: error: ‘constexpr C::C()’ called in a constant expression
 constexpr C d = {}; //error
                  ^
t.cc:1:8: note: ‘constexpr C::C()’ is not usable as a constexpr function
because:
 struct C
        ^
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idG007

Tobias Burnus <burnus at gcc dot gnu.org> changed:

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

--- Comment #18 from Tobias Burnus <burnus at gcc dot gnu.org> ---
Given that we already have some special handling for DECIMAL="comma", I wonder
whether one couldn't simply undo the setting by used, e.g.,

  const char *decimal;
  size_t decimal_len;

#if defined USE_LOCALECONV && !defined USE_NL_LANGINFO
  const struct lconv *lc = localeconv ();
#endif

#ifdef USE_NL_LANGINFO
  decimal = nl_langinfo (DECIMAL_POINT);
  decimal_len = strlen (decimal);
  assert (decimal_len > 0);
#elif defined USE_LOCALECONV
  decimal = lc->decimal_point;
  if (decimal == NULL || *decimal == '\0')
    decimal = ".";
  decimal_len = strlen (decimal);
#else
  decimal = ".";
  decimal_len = 1;
#endif

And then simply replace for decimal_len the existing ("." or ",") decimal
character from the read string/file by the one obtained for the local - before
sending it to strtof/strtod/strtold/strtoflt128.

(Note: decimal_len is probably nearly always == 1, but it might be different.)

[Bug c++/57820] Can't initialize a constexpr object with inline initialization

[harald at gigawatt dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

Harald van Dijk <harald at gigawatt dot nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |harald at gigawatt dot nl

--- Comment #2 from Harald van Dijk <harald at gigawatt dot nl> ---
Adding an user-defined constexpr C() {} makes it fail more clearly:

struct C
{
    int a = 2;
    int b = a + 1;

    constexpr C() {}
};


test.cc: In constructor ‘constexpr C::C()’:
test.cc:6:20: sorry, unimplemented: use of the value of the object being
constructed in a constant expression
     constexpr C() {}

and indeed, if `int b = a + 1;` is changed to `int b = 3;`, G++ accepts it.
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From: "dean at arctic dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/29776] result of ffs/clz/ctz/popcount/parity are already sign-extended
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id)776

--- Comment #10 from dean at arctic dot org ---
On Thu, 4 Jul 2013, jakub at gcc dot gnu.org wrote:

> I'm not 100% sure about CLZ/CTZ in the patch, because it could return any value
> for argument of 0, but as we document it as undefined behavior, perhaps it is
> fine.

this is unfortunate really -- the newer LZCNT/TZCNT instructions on x86
explicitly return 8*sizeof(type) for input of zero because the undefined
behaviour of BSF/BSR posed a lot of problems in inner loops.  it might be
useful to add explicit builtins for this behaviour.

thanks for taking a look at the bug :)

-dean

[Bug c++/57820] Can't initialize a constexpr object with inline initialization

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |accepts-invalid

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
AFAICS, in mainline this is now an accepts-invalid, because we accept
submitter's testcase too as-is.

I'm going to add to the testsuite the valid variants in Comments 1 & 2.

[Bug c++/57820] Can't initialize a constexpr object with inline initialization

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

--- Comment #4 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Mon Nov  3 10:49:05 2014
New Revision: 217035

URL: https://gcc.gnu.org/viewcvs?rev=217035&root=gcc&view=rev
Log:
2014-11-03  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/57820
    * g++.dg/cpp0x/constexpr-ctor16.C: New.
    * g++.dg/cpp0x/constexpr-ctor17.C: Likewise.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor16.C
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-ctor17.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/57820] [DR 253] NSDMI and const objects

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |SUSPENDED
                 CC|                            |jason at gcc dot gnu.org
            Summary|Can't initialize a          |[DR 253] NSDMI and const
                   |constexpr object with       |objects
                   |inline initialization       |

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
This issue is DR 253, which has not been resolved, but the committee seems
inclined to allow this code, so I'm not going to change G++ at this time.

http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_toc.html#253

[Bug c++/57820] [DR 253] NSDMI and const objects

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57820

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|SUSPENDED                   |NEW

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
[Adopted at the November, 2016 meeting as part of paper P0490R0.]