[Bug c++/49505] New: [DR 214] wrong partial ordering for explicit instantiation with unused template parameter

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49505

           Summary: [DR 214] wrong partial ordering for explicit
                    instantiation with unused template parameter
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: ja...@gcc.gnu.org


template <typename T> T f(int) { return T(); }
template <typename U, typename T, typename V> T f(V);

template int f<int> (int);

Here, the first f is more specialized than the second.  G++ currently rejects
this testcase as ambiguous because of the unused template parameter U, but
since DR 214 14.8.2.4 says,

In most cases, all template parameters must have values in order for deduction
to succeed, but for partial ordering purposes a template parameter may remain
without a value provided it is not used in the types being used for partial
ordering.

clang and EDG get this right.

I noticed this issue while looking at bug 22596.