[Bug c++/11548] New: [3.4 regression] Forwarding default template args ICEs

[bangerth at dealii dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11548

           Summary: [3.4 regression] Forwarding default template args ICEs
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          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

This was recently broken:
---------------------------------
template <typename> struct length {
    static const unsigned int value = 0;
};

template <typename T, int N = length<T>::value> class C;
template <typename T>                           class C<T, 0> {};

template <typename> C<int> foo ();
----------------------------------
Note that we need to look at the default argument of the general
template to realize that the value of N in C<int> equals zero, and
then take the partial specialization of C<T,0>. It's a funny idiom (used
in boost, for example), but it shouldn't ICE:

tmp/gg> ../build-gcc/gcc-install/bin/c++ -c x.cc
x.cc:8: internal compiler error: Segmentation fault
Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://gcc.gnu.org/bugs.html> for instructions.

A backtrace in the compiler is this:
#0  convert_nontype_argument (type=0x4001e57c, expr=0x40023348)
    at ../../gcc/gcc/cp/pt.c:2999
#1  0x080734bb in convert_template_argument (parm=0x403d9ca8, arg=0x40023348, 
    args=0x40023330, complain=tf_error, i=1, in_decl=0x403d9f30)
    at ../../gcc/gcc/cp/pt.c:3574
#2  0x08073619 in coerce_template_parms (parms=0x400232e8, args=0x2, 
    in_decl=0x403d9f30, complain=tf_error, require_all_arguments=1)
    at ../../gcc/gcc/cp/pt.c:3676
#3  0x08074ad8 in lookup_template_class (d1=0x403dc118, arglist=0x403dc410, 
    in_decl=0x0, context=0x0, entering_scope=0, complain=tf_error)
    at ../../gcc/gcc/cp/pt.c:4175
#4  0x080c3867 in finish_template_type (name=0x4001e57c, args=0x4001e57c, 
    entering_scope=1073866108) at ../../gcc/gcc/cp/semantics.c:2172

The crash is in the if()-clause of this snippet, since expr_type==0:
  /* An integral constant-expression can include const variables or
     enumerators.  Simplify things by folding them to their values,
     unless we're about to bind the declaration to a reference
     parameter.  */
  if (INTEGRAL_TYPE_P (expr_type)
      && TREE_CODE (type) != REFERENCE_TYPE)
    expr = decl_constant_value (expr);

None of the code in the lowest 3 frames seems to have been changed
recently, so I can't tell which the breaking patch is. Unfortunately,
I also can't see a work-around for the problem, short of actually
writing in the value of the default argument at the point of use :-(

W.