[Bug c++/52902] New: Pointer to member rejected in constant expression

[Bug c++/52902] New: Pointer to member rejected in constant expression

[freunddeslichts at web dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52902

             Bug #: 52902
           Summary: Pointer to member rejected in constant expression
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: freunddeslichts@web.de


#########################################################################


struct X {
  int a;
};

constexpr int X::* mem = &X::a;

void foo() 
{
  constexpr X x = {1};
  constexpr int k = x.a;          // ok

  constexpr int l = x.*mem;       // should be ok       <------------------

  //constexpr int const* m = &(x.a);  // legitimate error
}


#########################################################################

g++-4.7 -std=c++0x bug.cpp -o g47.out
bug.cpp: In function ‘void foo()’:
bug.cpp:29:24: error: ‘(const int*)(& x)’ is not a constant expression

#########################################################################


As far as I can see, a pointer to member access should be a constant
expresssion, iff both the object and the pointer are constant expressions. 
This behaviour is shown by the clang++ compiler, which does not emit the above
error.


It seems gcc evalutates the expression 

         x.*mem

as something like

         *((const int*)(& x) + mem)

In this implementation, the left (intermediate) operand to the + operator is
not a constant expression, that's why gcc rejects it by mistake.

[Bug c++/52902] Pointer to member rejected in constant expression

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52902

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ldionne.2 at gmail dot com

--- Comment #1 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
*** Bug 67376 has been marked as a duplicate of this bug. ***

[Bug c++/52902] Pointer to member rejected in constant expression

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52902

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-09-04
                 CC|                            |trippels at gcc dot gnu.org
             Blocks|                            |55004
     Ever confirmed|0                           |1


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55004
[Bug 55004] [meta-bug] constexpr issues

[Bug c++/52902] Pointer to member rejected in constant expression

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52902

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

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

--- Comment #2 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
Works with gcc-5 and trunk. Closing.

[Bug c++/52902] Pointer to member rejected in constant expression

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52902

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

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