[Bug c++/54002] New: [C++0x][constexpr] Initializing constexpr static member using constexpr static method fails

[Bug c++/54002] New: [C++0x][constexpr] Initializing constexpr static member using constexpr static method fails

[Martin.vGagern at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

             Bug #: 54002
           Summary: [C++0x][constexpr] Initializing constexpr static
                    member using constexpr static method fails
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Martin.vGagern@gmx.net


I would expect the following code to work with -std=gnu++0x, but it does not:

class C1 {
  constexpr static int foo(int x) { return x + 1; }
  constexpr static int bar = foo(sizeof(int));
};

g++ 4.5 complains:
error: ‘static int C1::foo(int)’ cannot appear in a constant-expression
error: a function call cannot appear in a constant-expression

g++ 4.6 and 4.7 complain:
error: field initializer is not constant



Detailed analysis of this problem follows below.

As the function is declared to be constexpr, this surprises me. I took this to
Stack Overflow to get some insight:
http://stackoverflow.com/q/11522399/1468366

In his answer http://stackoverflow.com/a/11523155/1468366, Ben Voigt quoted
relevant parts from the spec. The most likely cause for this *not* being a
constant expression appears to be section 5.19 paragraph 2 item 3:
"an invocation of an undefined constexpr function […]"
qutoed from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf

But the definition is there, right in the class. Neither of us could find a
part in the specification which might cause the function to not be defined at
that point, particularly as the class is completely defined within the
initializer, according to section 9.2 paragraph 2.

In case there actually is some part of the spec which makes gccs current
behaviour the only correct one, then please consider this report here as a
request for a more useful error message, which might make the actual problem
known without having to read the specs for hours.

If, on the other hand, the specs allow code like the one above, as I believe
they do, then please try to make gcc accept that code as well.

[Bug c++/54002] [C++0x][constexpr] Initializing constexpr static member using constexpr static method fails

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

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

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-07-17 20:04:59 UTC ---
N.B. 4.5 doesn't implement constexpr (it parses the keyword and pretty much
ignores it) so that error is irrelevant

I think this might be because C1::foo can't be used in a constant expression
until C1 is complete. Jason?

[Bug c++/54002] [C++0x][constexpr] Initializing constexpr static member using constexpr static method fails

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-07-17 20:06:07 UTC ---
See PR 52366, which makes this a dup of PR 52315

[Bug c++/54002] [C++0x][constexpr] Initializing constexpr static member using constexpr static method fails

[Martin.vGagern at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

--- Comment #3 from Martin von Gagern <Martin.vGagern at gmx dot net> 2012-07-18 07:09:02 UTC ---
(In reply to comment #2)
> See PR 52366, which makes this a dup of PR 52315

I agree that this is a duplicate of PR 52366. And although I'm still not
convinced that this is really the same requirement as the one in PR 52315, I'm
inclined to believe it nevertheless. Should PR 52366 be un-duped, like PR 52366
comment 4 requested out, then this here should still be duped to PR 52366
instead of PR 52315, so I suggest duping it there in the first place.

[Bug c++/54002] [C++0x] Initializing constexpr static member using constexpr static method fails

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
I'm adding the testcase to the testsuite and closing the bug.

[Bug c++/54002] [C++0x] Initializing constexpr static member using constexpr static method fails

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

--- Comment #5 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Fri Aug 29 15:58:26 2014
New Revision: 214734

URL: https://gcc.gnu.org/viewcvs?rev=214734&root=gcc&view=rev
Log:
2014-08-29  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/54002
    * g++.dg/cpp0x/constexpr-54002.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-54002.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/54002] [C++0x] Initializing constexpr static member using constexpr static method fails

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54002

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.