[Bug c++/85242] New: [6/7/8 Regression] ICE with invalid template parameter

["reichelt at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85242

            Bug ID: 85242
           Summary: [6/7/8 Regression] ICE with invalid template parameter
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: ice-on-invalid-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reichelt at gcc dot gnu.org
  Target Milestone: ---

The following invalid code snippet triggers an ICE on trunk since GCC 6.1.0:

==========================================
namespace N
{
  struct A {};
}

template<struct N::A {}> void foo();
==========================================

'
in dump_template_decl, at cp/error.c:1305
 template<struct N::A {}> void foo();
                    ^
0x7163e0 dump_template_decl
        ../../gcc-6.1.0/gcc/cp/error.c:1305
0x718bf2 decl_to_string
        ../../gcc-6.1.0/gcc/cp/error.c:2955
0x718bf2 cp_printer
        ../../gcc-6.1.0/gcc/cp/error.c:3535
0x14b50b8 pp_format(pretty_printer*, text_info*)
        ../../gcc-6.1.0/gcc/pretty-print.c:631
0x14b0928 diagnostic_report_diagnostic(diagnostic_context*, diagnostic_info*)
        ../../gcc-6.1.0/gcc/diagnostic.c:823
0x14b17ed error(char const*, ...)
        ../../gcc-6.1.0/gcc/diagnostic.c:1152
0x644646 duplicate_decls(tree_node*, tree_node*, bool)
        ../../gcc-6.1.0/gcc/cp/decl.c:1609
0x81664c pushdecl_maybe_friend_1
        ../../gcc-6.1.0/gcc/cp/name-lookup.c:805
0x81664c pushdecl_maybe_friend(tree_node*, bool)
        ../../gcc-6.1.0/gcc/cp/name-lookup.c:1298
0x819753 pushdecl_with_scope_1
        ../../gcc-6.1.0/gcc/cp/name-lookup.c:2295
0x81983c pushdecl_with_scope(tree_node*, cp_binding_level*, bool)
        ../../gcc-6.1.0/gcc/cp/name-lookup.c:2309
0x8198ef pushdecl_namespace_level(tree_node*, bool)
        ../../gcc-6.1.0/gcc/cp/name-lookup.c:3915
0x6b5e30 push_template_decl_real(tree_node*, bool)
        ../../gcc-6.1.0/gcc/cp/pt.c:5392
0x72f2da cp_parser_class_head
        ../../gcc-6.1.0/gcc/cp/parser.c:21951
0x72f2da cp_parser_class_specifier_1
        ../../gcc-6.1.0/gcc/cp/parser.c:21285
0x72f2da cp_parser_class_specifier
        ../../gcc-6.1.0/gcc/cp/parser.c:21582
0x72f2da cp_parser_type_specifier
        ../../gcc-6.1.0/gcc/cp/parser.c:15836
0x743571 cp_parser_decl_specifier_seq
        ../../gcc-6.1.0/gcc/cp/parser.c:12760
0x7451e6 cp_parser_parameter_declaration
        ../../gcc-6.1.0/gcc/cp/parser.c:20437
0x7467e9 cp_parser_template_parameter
        ../../gcc-6.1.0/gcc/cp/parser.c:14543
Please submit a full bug report, [etc.]

Note that the compiler even fails to properly emit the
"internal compiler error" message.
GCC 4.4 - GCC 5 wrongly accepted the code.