The following valid code snippet triggers an ICE since GCC 3.1: ==================================================================== template<template <typename T, typename = T > class U> struct A { template<int> U<int> foo(); }; ==================================================================== bug.cc:3: internal compiler error: in tsubst, at cp/pt.c:7450 Please submit a full bug report, [etc.] This is related to PR 30044, but here we have a regression.
Confirmed.
This code in lookup_template_class: - /* Consider an example where a template template parameter declared as - - template <class T, class U = std::allocator<T> > class TT - - The template parameter level of T and U are one level larger than - of TT. To proper process the default argument of U, say when an - instantiation `TT<int>' is seen, we need to build the full - arguments containing {int} as the innermost level. Outer levels, - available when not appearing as default template argument, can be - obtained from `current_template_args ()'. - - Suppose that TT is later substituted with std::vector. The above - instantiation is `TT<int, std::allocator<T> >' with TT at - level 1, and T at level 2, while the template arguments at level 1 - becomes {std::vector} and the inner level 2 is {int}. */ - - if (current_template_parms) - arglist = add_to_template_args (current_template_args (), arglist); is simply bogus; the current template arguments have nothing to do with the situation. It is this code which causes the crash. I believe that the correct change is to remove that code and to make template parameters for template template parameters restart at level 1. So, in: template <template <typename T> class A> class B {}; both T and A have level 1. (Right now, T has level 2.) T should have level 1 because, when instantiating A, that's what make sense.
By the way, the hunk of code at issue dates back to this time: http://gcc.gnu.org/ml/gcc-patches/2001-10/msg00589.html (and, by the way, I cannot find an explicit approval in the mailing list). Anyway, the effect of removing it is fixing this PR and regressing only on the testcase committed with it (template/ttp2.C).
Subject: Bug 30897 Author: jason Date: Fri Nov 2 05:50:06 2007 New Revision: 129844 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=129844 Log: PR c++/30897 * pt.c (push_template_decl_real): Set DECL_CONTEXT on template template parms. (lookup_template_class): Use it to get the outer template args for instantiating one. PR c++/29236 * pt.c (reduce_template_parm_level): tsubst the parameters of a template template parm. Added: trunk/gcc/testsuite/g++.dg/template/ttp23.C trunk/gcc/testsuite/g++.dg/template/ttp24.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/pt.c
Fixed for 4.3, though not as Mark suggested; we need to have the arguments of the enclosing templates in order to bind names in the default argument properly.
Closing 4.1 branch.
Closing 4.2 branch, fixed in 4.3.