[Bug c++/85263] New: [concepts] ICE with parameter pack matching

[reichelt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85263

            Bug ID: 85263
           Summary: [concepts] ICE with parameter pack matching
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: error-recovery, ice-on-invalid-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reichelt at gcc dot gnu.org
            Blocks: 67491
  Target Milestone: ---

The following invalid code snippet (compiled with "-fconcepts") triggers
an ICE since GCC 6.1.0:

=======================================================
template<typename..., int...> concept bool C = true;

C{A, B} void foo();
=======================================================

bug.cc:3:3: error: cannot match pack for introduced parameter
 C{A, B} void foo();
   ^
bug.cc:3:7: internal compiler error: tree check: accessed elt 1 of tree_vec
with 0 elts in process_introduction_parm, at cp/constraint.cc:1325
 C{A, B} void foo();
       ^
0x78f405 tree_vec_elt_check_failed(int, int, char const*, int, char const*)
        ../../gcc/gcc/tree.c:9520
0x5e94ec tree_vec_elt_check(tree_node*, int, char const*, int, char const*)
        ../../gcc/gcc/tree.h:3356
0x5e94ec process_introduction_parm
        ../../gcc/gcc/cp/constraint.cc:1325
0x5e94ec finish_template_introduction(tree_node*, tree_node*)
        ../../gcc/gcc/cp/constraint.cc:1408
0x944368 cp_parser_template_introduction
        ../../gcc/gcc/cp/parser.c:27066
0x944368 cp_parser_template_declaration_after_export
        ../../gcc/gcc/cp/parser.c:27170
0x949714 cp_parser_declaration
        ../../gcc/gcc/cp/parser.c:12767
0x949b31 cp_parser_declaration_seq_opt
        ../../gcc/gcc/cp/parser.c:12648
0x949e24 cp_parser_translation_unit
        ../../gcc/gcc/cp/parser.c:4563
0x949e24 c_parse_file()
        ../../gcc/gcc/cp/parser.c:39029
0xa49bb6 c_common_parse_file()
        ../../gcc/gcc/c-family/c-opts.c:1132
Please submit a full bug report, [etc.]


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67491
[Bug 67491] [meta-bug] concepts issues