[Bug c++/105725] New: [ICE] segfault with `-Wmismatched-tags`

["tim at klingt dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105725

            Bug ID: 105725
           Summary: [ICE] segfault with `-Wmismatched-tags`
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tim at klingt dot org
  Target Milestone: ---

possibly a duplicate of 103703, but the backtrace is slightly different, so i'd
file a separate ticket for it:

cwise gave me:
```
template <class T> class T::QPrivateSignal
```

stack trace
```
0xd8e7d3 crash_signal
        ../../src/gcc/toplev.cc:322
0x7f59c77a051f ???
        ./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
0x7ba59e class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*,
bool, bool)
        ../../src/gcc/cp/parser.cc:33489
0x7e44ba cp_parser_elaborated_type_specifier
        ../../src/gcc/cp/parser.cc:20659
0x7c8972 cp_parser_type_specifier
        ../../src/gcc/cp/parser.cc:19370
0x7d8e90 cp_parser_type_specifier_seq
        ../../src/gcc/cp/parser.cc:24349
0x7d7214 cp_parser_type_id_1
        ../../src/gcc/cp/parser.cc:24149
0x7d9fb3 cp_parser_template_type_arg
        ../../src/gcc/cp/parser.cc:24257
0x7da119 cp_parser_template_argument
        ../../src/gcc/cp/parser.cc:18799
0x7da119 cp_parser_template_argument_list
        ../../src/gcc/cp/parser.cc:18723
0x7da119 cp_parser_enclosed_template_argument_list
        ../../src/gcc/cp/parser.cc:32029
0x7db50f cp_parser_template_id
        ../../src/gcc/cp/parser.cc:18262
0x7dbcc9 cp_parser_class_name
        ../../src/gcc/cp/parser.cc:25698
0x7d386e cp_parser_qualifying_entity
        ../../src/gcc/cp/parser.cc:7116
0x7d386e cp_parser_nested_name_specifier_opt
        ../../src/gcc/cp/parser.cc:6798
0x7eaa4c cp_parser_simple_type_specifier
        ../../src/gcc/cp/parser.cc:19761
0x7c89d5 cp_parser_type_specifier
        ../../src/gcc/cp/parser.cc:19420
0x7d8e90 cp_parser_type_specifier_seq
        ../../src/gcc/cp/parser.cc:24349
0x7d7214 cp_parser_type_id_1
        ../../src/gcc/cp/parser.cc:24149
0x7d9fb3 cp_parser_template_type_arg
        ../../src/gcc/cp/parser.cc:24257
```