[Bug c++/97536] New: [concepts] parser segfault for concept defined in function template

["hello at josephloser dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97536

            Bug ID: 97536
           Summary: [concepts] parser segfault for concept defined in
                    function template
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hello at josephloser dot com
  Target Milestone: ---

The code below causes a segfault when parsing a concept definition inside a
function template. Note that concepts cannot be defined inside a function
scope, so this should result in a useful diagnostic.

Output of compiling the program with -std=c++2a:
prog.cc: In function 'auto foo()':
prog.cc:3:5: warning: C++20 concept definition syntax is 'concept <name> =
<expr>'
    3 |     concept bar = true;
      |     ^~~~~~~
prog.cc:3:13: internal compiler error: Segmentation fault
    3 |     concept bar = true;
      |             ^~~
0xc2e30f crash_signal
        ../../source/gcc/toplev.c:330
0x68c49c grokvardecl
        ../../source/gcc/cp/decl.c:10103
0x68c49c grokdeclarator(cp_declarator const*, cp_decl_specifier_seq*,
decl_context, int, tree_node**)
        ../../source/gcc/cp/decl.c:13691
0x6913c8 start_decl(cp_declarator const*, cp_decl_specifier_seq*, int,
tree_node*, tree_node*, tree_node**)
        ../../source/gcc/cp/decl.c:5218
0x71cc3e cp_parser_init_declarator
        ../../source/gcc/cp/parser.c:20934
0x6fe49c cp_parser_simple_declaration
        ../../source/gcc/cp/parser.c:13876
0x7002ad cp_parser_declaration_statement
        ../../source/gcc/cp/parser.c:13316
0x700e97 cp_parser_statement
        ../../source/gcc/cp/parser.c:11562
0x701e6d cp_parser_statement_seq_opt
        ../../source/gcc/cp/parser.c:11928
0x701f20 cp_parser_compound_statement
        ../../source/gcc/cp/parser.c:11878
0x718d2a cp_parser_function_body
        ../../source/gcc/cp/parser.c:23236
0x718d2a cp_parser_ctor_initializer_opt_and_function_body
        ../../source/gcc/cp/parser.c:23287
0x71c730 cp_parser_function_definition_after_declarator
        ../../source/gcc/cp/parser.c:29182
0x71d64f cp_parser_function_definition_from_specifiers_and_declarator
        ../../source/gcc/cp/parser.c:29098
0x71d64f cp_parser_init_declarator
        ../../source/gcc/cp/parser.c:20838
0x722c14 cp_parser_single_declaration
        ../../source/gcc/cp/parser.c:29669
0x722d77 cp_parser_template_declaration_after_parameters
        ../../source/gcc/cp/parser.c:29242
0x72337a cp_parser_explicit_template_declaration
        ../../source/gcc/cp/parser.c:29507
0x726cd9 cp_parser_declaration
        ../../source/gcc/cp/parser.c:13524
0x72726d cp_parser_translation_unit
        ../../source/gcc/cp/parser.c:4793
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For what its worth, the warning given for when a concept is defined inside a
function (not a function template) could probably be improved as well. If you
remove the template on `foo` such that it is just a function, the error is:

prog.cc: In function 'auto foo()':
prog.cc:2:5: warning: C++20 concept definition syntax is 'concept <name> =
<expr>'
    2 |     concept bar = true;
      |     ^~~~~~~
prog.cc:2:5: error: a non-template variable cannot be 'concept'

The error message is fine, but the warning is a bit of a misnomer as `concept
bar = true;` does meet the grammar of `concept <name> = <expr>`.