[Bug c++/17570] New: Extension to incorporate default parameters in signature of templates breaks valid program

["bangerth at dealii dot org" <gcc-bugzilla at gcc dot gnu dot org>]

We have an extension by which a template class X with n parameters can 
match a template template parameter with m<n parameters, if X 
provides default arguments for the remaining n-m template parameters. 
 
Unfortunately, this extension leads us to reject this (valid) program: 
-------------------- 
template<typename> struct match { }; 
 
template<template<typename> class t,typename T>  
struct match<t<T> > { typedef int type; }; 
 
template<template<typename,typename> class t,typename T0,typename T1> 
struct match<t<T0,T1> > { typedef int type; }; 
 
template<typename,typename =void> struct other { }; 
 
typedef match<other<void,void> >::type type; 
--------------------------- 
 
g/x> /home/bangerth/bin/gcc-4*/bin/c++ -c x.cc 
x.cc:11: error: ambiguous class template instantiation for `struct 
match<other<void, void> >' 
x.cc:7: error: candidates are: struct match<t<T0, T1> > 
x.cc:4: error:                 struct match<t<T> > 
x.cc:11: error: `type' in class `match<other<void, void> >' does not name a 
type 
 
I think we should first try to match the two-template parameter version 
of "other", and _only if that fails_, try to also use the default argument 
and match it against one-parameter template template parameters. 
 
W.

-- 
           Summary: Extension to incorporate default parameters in signature
                    of templates breaks valid program
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bangerth at dealii dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17570