[Bug c++/57979] New: G++ accepts constant expression defined using floating point glvalue

[Bug c++/57979] New: G++ accepts constant expression defined using floating point glvalue

[jogojapan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

            Bug ID: 57979
           Summary: G++ accepts constant expression defined using floating
                    point glvalue
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jogojapan at gmail dot com

The following code is accepted and compiled without error by G++:

int main()
{
  const float f1 = 0.0;

  constexpr float f2 = f1;

  return 0;
}

Note that the declaration of f2 as constexpr requires f1 to be a constant
expression, which it isn't, according to §5.19/2:

====(QUOTE)========
A condition-expression is a core constant expression unless it involves
[...] 
[..9th item..]
 - an lvalue-to-rvalue conversion (4.1) unless it is applied to [...] a glvalue
of integral or enumeration type that refers to a non-volatile const object with
a preceding initialization, initialized with a constant expresion, or
[...]
====(ENDQUOTE)====

So if f1 had been declared as const int, the code would be acceptable, but
there is no such exception for floating point (or any other non-integral or
non-enum type).


Tested with a 20130708 snapshot checkout from the master branch of the
git-mirror of the repository. (I'll test with a more recent version soon.)
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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-07-25
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's accepted even with -pedantic

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[jogojapan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

--- Comment #2 from Johannes Goller <jogojapan at gmail dot com> ---
Confirmed that the problem exists in the most recent version of GCC 4.9, i.e. a
20130725 snapshot from the master branch.

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ville.voutilainen at gmail dot com

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
*** Bug 59676 has been marked as a duplicate of this bug. ***

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

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

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

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
This was introduced by the fix for bug 21089; decay_conversion and
convert_like_real shouldn't be pulling values out of variables that are not
decl_const_var_p.  The trick is fixing that without breaking the static
initialization optimization that 21089 is about.  I guess allowing
maybe_constant_init to be more aggressive than *_constant_value is the way to
approach this.

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

--- Comment #5 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
I think this should be put on hold, since EWG told CWG in Rapperswil
that const floats should perform the same magic as const ints do, see
http://open-std.org/JTC1/SC22/WG21/docs/cwg_closed.html#1826
which EWG suggested CWG to reopen and resolve.

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

--- Comment #6 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
In other words, EWG wants the standard to be changed so that GCC's behavior
actually becomes conforming, as far as I understand the intent.

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Wed Nov 19 22:06:17 2014
New Revision: 217814

URL: https://gcc.gnu.org/viewcvs?rev=217814&root=gcc&view=rev
Log:
    PR c++/57979
    * init.c (decl_really_constant_value): Rename from
    integral_constant_value.
    (scalar_constant_value): Similar but limited to scalar results.
    (decl_constant_value_safe): Remove.
    (constant_value_1): Respect return_aggregate_cst_ok_p.
    * typeck.c (decay_conversion): Use scalar_constant_value.
    * call.c (convert_like_real): Likewise.
    * cvt.c (ocp_convert): No need to check CLASS_TYPE_P.
    * typeck.c (decay_conversion): Or ARRAY_TYPE.
    * constexpr.c (struct constexpr_ctx): Add strict field.
    (cxx_eval_constant_expression) [VAR_DECL]: Use it to select between
    decl_constant_value and decl_really_constant_value.
    (cxx_eval_outermost_constant_expr): Add strict parm.
    (maybe_constant_init): Not strict.
    (potential_constant_expression_1): Add strict parm.
    Shorten most internal calls with RECUR macro.
    * cp-tree.h, pt.c, semantics.c: Adjust.

Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/cp/constexpr.c
    trunk/gcc/cp/cp-tree.h
    trunk/gcc/cp/cvt.c
    trunk/gcc/cp/decl.c
    trunk/gcc/cp/init.c
    trunk/gcc/cp/pt.c
    trunk/gcc/cp/semantics.c
    trunk/gcc/cp/typeck.c
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-object2.C
    trunk/gcc/testsuite/g++.dg/cpp0x/nullptr06.C

[Bug c++/57979] G++ accepts constant expression defined using floating point non-constexpr glvalue

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57979

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
   Target Milestone|---                         |5.0

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Ville Voutilainen from comment #5)
> I think this should be put on hold, since EWG told CWG in Rapperswil
> that const floats should perform the same magic as const ints do, see
> http://open-std.org/JTC1/SC22/WG21/docs/cwg_closed.html#1826
> which EWG suggested CWG to reopen and resolve.

I had already done most of the work before you made this comment.  :)

You might want to ping Mike about that again, as I don't think it came up in
core at the Urbana meeting.

Fixed for gcc 5.