[Bug c++/71543] New: [concepts] ICE on ill-formed declaration of a parameter with a constrained-type-specifier in a requires expression

["tom at honermann dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71543

            Bug ID: 71543
           Summary: [concepts] ICE on ill-formed declaration of a
                    parameter with a constrained-type-specifier in a
                    requires expression
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tom at honermann dot net
                CC: andrew.n.sutton at gmail dot com, asutton at gcc dot gnu.org
  Target Milestone: ---

The following ill-formed code triggers an ICE in gcc trunk r236947.  The code
is ill-formed because constrained-type-specifiers are not permitted to declare
parameters in requires expressions.

$ cat t.cpp
template<typename T>
concept bool C1 = true;
template<typename T>
concept bool C2 = requires(C1 c1) {};

$ svn info
Path: .
Working Copy Root Path: /home/tom/src/gcc-trunk
URL: svn://gcc.gnu.org/svn/gcc/trunk
Relative URL: ^/trunk
Repository Root: svn://gcc.gnu.org/svn/gcc
Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4
Revision: 236947
Node Kind: directory
Schedule: normal
Last Changed Author: jason
Last Changed Rev: 236947
Last Changed Date: 2016-05-31 15:49:22 -0400 (Tue, 31 May 2016)

$ gcc --version
gcc (GCC) 7.0.0 20160531 (experimental)
...

$ g++ -c -std=c++1z -fconcepts t.cpp 
t.cpp:4:28: internal compiler error: in synthesize_implicit_template_parm, at
cp/parser.c:37843
 concept bool C2 = requires(C1 c1) {};
                            ^~
0x753793 synthesize_implicit_template_parm
        ../../gcc-trunk/gcc/cp/parser.c:37843
0x7539cc cp_parser_maybe_constrained_type_specifier
        ../../gcc-trunk/gcc/cp/parser.c:16463
0x753af8 cp_parser_nonclass_name
        ../../gcc-trunk/gcc/cp/parser.c:16541
0x761d41 cp_parser_type_name
        ../../gcc-trunk/gcc/cp/parser.c:16347
0x761d41 cp_parser_simple_type_specifier
        ../../gcc-trunk/gcc/cp/parser.c:16261
0x75dbdd cp_parser_type_specifier
        ../../gcc-trunk/gcc/cp/parser.c:15914
0x773494 cp_parser_decl_specifier_seq
        ../../gcc-trunk/gcc/cp/parser.c:12758
0x775159 cp_parser_parameter_declaration
        ../../gcc-trunk/gcc/cp/parser.c:20448
0x7759a4 cp_parser_parameter_declaration_list
        ../../gcc-trunk/gcc/cp/parser.c:20263
0x775e44 cp_parser_parameter_declaration_clause
        ../../gcc-trunk/gcc/cp/parser.c:20184
0x7592db cp_parser_requirement_parameter_list
        ../../gcc-trunk/gcc/cp/parser.c:24391
0x7592db cp_parser_requires_expression
        ../../gcc-trunk/gcc/cp/parser.c:24362
0x7592db cp_parser_primary_expression
        ../../gcc-trunk/gcc/cp/parser.c:5098
0x762775 cp_parser_postfix_expression
        ../../gcc-trunk/gcc/cp/parser.c:6688
0x760b5c cp_parser_unary_expression
        ../../gcc-trunk/gcc/cp/parser.c:7986
0x76ad47 cp_parser_cast_expression
        ../../gcc-trunk/gcc/cp/parser.c:8663
0x76b333 cp_parser_binary_expression
        ../../gcc-trunk/gcc/cp/parser.c:8764
0x76bc24 cp_parser_assignment_expression
        ../../gcc-trunk/gcc/cp/parser.c:9051
0x76c06a cp_parser_constant_expression
        ../../gcc-trunk/gcc/cp/parser.c:9321
0x76c8c4 cp_parser_initializer_clause
        ../../gcc-trunk/gcc/cp/parser.c:20837
...