[Bug c++/54020] New: [c++0x] incorrectly accepted constexpr functions

[Bug c++/54020] New: [c++0x] incorrectly accepted constexpr functions

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

             Bug #: 54020
           Summary: [c++0x] incorrectly accepted constexpr functions
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: luto@mit.edu


Tested on a somewhat recent trunk build as well as 4.7-some branch version.

// Preliminaries.
extern int nonconst_func(int);
constexpr int identity(int x) { return x; }
constexpr int zero() { return identity(0); }
constexpr int one() { return identity(1); }

// Correctly accepted.
constexpr int three = one() ? 3 : nonconst_func(0);

// Incorrectly accepted.  See [dcl.constexpr] #5:
//   For a constexpr function, if no function argument values exist
//   such that the function invocation sub-stitution would produce a
//   constant expression (5.19), the program is ill-formed; no diagnostic
//   required.
constexpr int bogus() { return zero () ? 3 : nonconst_func(0); }

// Correctly rejected (not sure why).
constexpr int correct_error() { return nonconst_func(0); }

// Correctly rejected.
constexpr int z = bogus();

// This is also correctly rejected.
constexpr int correct_failure() { return 0 ? 3 : nonconst_func(0); }

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-07-19 06:40:41 UTC ---
... the program is ill-formed; no diagnostic required.
                               ^^^^^^^^^^^^^^^^^^^^^^^
GCC doesn't immediately see that the first operand of ?: is zero or non-zero in
that function, so assumes it could be either zero or non-zero and doesn't
diagnose, you get diagnostics only when you are actually using the function in
some constexpr var initializer or other context where a constant expression is
required.

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

--- Comment #2 from Andy Lutomirski <luto at mit dot edu> 2012-07-19 15:41:51 UTC ---
I clearly failed at reading comprehension yesterday.

Maybe this should be considered as more of an enhancement request (like
PR54021): it would be nicer for the user if constexpr worked the same with an
without optimization.  Otherwise there'll probably be reports of code that
builds at -O2 but not -O0.

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

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

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

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-07-25 09:39:36 UTC ---
Let's add Jason in CC (about the -O0 vs -O2 thing, in particular)

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2012-07-25
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2012-07-25 13:53:22 UTC ---
This testcase behaves the same at -O0 or -O2.  Seems like a simple enough
enhancement, though.

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2012-07-25 14:57:01 UTC ---
Author: jason
Date: Wed Jul 25 14:56:57 2012
New Revision: 189851

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=189851
Log:
    PR c++/54020
    * semantics.c (potential_constant_expression_1) [COND_EXPR]: Call
    maybe_constant_value.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-neg2.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c

[Bug c++/54020] [c++0x] incorrectly accepted constexpr functions

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54020

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.8.0
           Severity|normal                      |enhancement

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> 2012-07-25 15:04:27 UTC ---
Fixed for 4.8.