[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.