[Bug c++/48617] New: [C++0x] Problem with non-type template parameters and decltype

["daniel.kruegler at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48617

           Summary: [C++0x] Problem with non-type template parameters and
                    decltype
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: daniel.kruegler@googlemail.com


gcc 4.7.0 in C++0x mode rejects the following program:

//---------------
template<class T, decltype(T())> // #
struct A {};

A<int, 0> a;

int main() {}
//---------------

The error message annotated at line # is:

"error: 'decltype (T())' is not a valid type for a template constant parameter"

It seems to me that according to the FDIS this should be well-formed program:
decltype is feasible as a /parameter-declaration/. The new type-equivalence
rules in [temp.type] p. 2,

"If an expression e involves a template parameter, decltype(e) denotes a unique
dependent type. Two such decltype-specifiers refer to the same type only if
their expressions are equivalent (14.5.6.1)."

should allow for a compiler to deduce the meaning similar to other dependent
contexts. See also [temp.dep.type] p. 8 for this matter:

"A type is dependent if it is [...] denoted by decltype(expression), where
expression is type-dependent (14.6.2.2)."