[Bug c++/54912] New: [C++11] Non-type argument to alias template is not a constant expression

[Bug c++/54912] New: [C++11] Non-type argument to alias template is not a constant expression

[lucdanton at free dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54912

             Bug #: 54912
           Summary: [C++11] Non-type argument to alias template is not a
                    constant expression
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: lucdanton@free.fr


gcc version 4.8.0 20121008 (experimental) with flags

-Wall -pedantic -std=c++11

complains when fed the following program:

template<bool B>
using Bool = int;

template<typename T>
void foo()
{
    using bar = Bool<(sizeof(T) >= 0)>; // line 7
}

int main()
{
    foo<int>();
}

Compiler output:

main.cpp: In substitution of 'template<bool B> using Bool = int [with bool B =
(sizeof (T) >= 0)]':
main.cpp:7:38:   required from here
main.cpp:7:38: error: integral expression '(sizeof (T) >= 0)' is not constant
     using bar = Bool<(sizeof(T) >= 0)>;
                                      ^
main.cpp:7:38: error:   trying to instantiate 'template<bool B> using Bool =
int'

Using the sizeof expression as the non-type parameter of a class template with
the same template parameter list as the alias template is accepted. That same
expression can also be used to initialize e.g. a constexpr bool variable, which
will in fact in turn be accepted as a parameter to Bool.

Previous snapshots of GCC used to accept this kind of code.

[Bug c++/54912] [4.8 Regression] Non-type argument to alias template is not a constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54912

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-12
            Summary|[C++11] Non-type argument   |[4.8 Regression] Non-type
                   |to alias template is not a  |argument to alias template
                   |constant expression         |is not a constant
                   |                            |expression
     Ever Confirmed|0                           |1

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-12 16:54:53 UTC ---
Confirmed as a regression, 4_7-branch is fine.

[Bug c++/54912] [4.8 Regression] Non-type argument to alias template is not a constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54912

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

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

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-12 17:07:56 UTC ---
Most likely a Dup of PR54859, also regressed with 191412.

[Bug c++/54912] [4.8 Regression] Non-type argument to alias template is not a constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54912

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pkeir at outlook dot com

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-21 22:51:44 UTC ---
*** Bug 55013 has been marked as a duplicate of this bug. ***

[Bug c++/54912] [4.8 Regression] Non-type argument to alias template is not a constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54912

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

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

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-21 22:53:20 UTC ---
Dup.

*** This bug has been marked as a duplicate of bug 54859 ***