[Bug c++/99850] New: [P1102R2] reject valid lambda syntax in C++23

[Bug c++/99850] New: [P1102R2] reject valid lambda syntax in C++23

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

            Bug ID: 99850
           Summary: [P1102R2] reject valid lambda syntax in C++23
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/x5E5cGPTb

auto l = []<auto> requires true -> void {};

gcc incorrectly rejects this valid lambda, you can see more details in
https://stackoverflow.com/questions/66833334/the-validity-of-lambda-expression-with-omitted-parameter-list-in-c23

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2021-03-31
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.  I could take a look, unless Jakub wants it.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Are you sure it is incorrectly rejected?
http://eel.is/c++draft/expr.prim.lambda.general
says:
 lambda-declarator:
   lambda-specifiers
   ( parameter-declaration-clause ) lambda-specifiers requires-clause[opt]
So in my reading, if requires-clause is present, the ()s are not optional.
Otherwise requires-clause[opt] would need to be in the lambda-specifiers
non-terminal or present also after the first lambda-specifiers.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Ah, but in lambda-expression: non-terminal there is another
requires-clause[opt] after the < template-parameter-list >.
So we need to handle
auto l = []<class T> requires true (T t, int n) { };
But during parsing we have parsing of requires clause at that spot after >,
and when ()s are omitted, there is code to handle -> at that spot.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

--- Comment #4 from 康桓瑋 <hewillk at gmail dot com> ---
This ICE may be caused by not handle this form.

template <typename...> concept C = true;

auto l = []<typename... Ts> requires (C<Ts> && ...) -> void {};

https://godbolt.org/z/vo8xPd4hY

<source>:3:48: internal compiler error: Segmentation fault
    3 | auto l = []<typename... Ts> requires (C<Ts> && ...) -> void {};
      |                                                ^~~
0x1cfca39 internal_error(char const*, ...)
        ???:0
0x940286 convert_generic_types_to_packs(tree_node*, int, int)
        ???:0
0x8e126d c_parse_file()
        ???:0
0xa60292 c_common_parse_file()
        ???:0
Please submit a full bug report,
with preprocessed source if appropriate.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 50592
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50592&action=edit
gcc11-pr99850.patch

Untested fix.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:784de5292c34e287c848b382b431599b818ea76e

commit r11-8210-g784de5292c34e287c848b382b431599b818ea76e
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Apr 16 09:34:26 2021 +0200

    c++: Fix up C++23 [] <...> requires primary -> type {} parsing [PR99850]

    The requires clause parsing has code to suggest users wrapping
    non-primary expressions in (), so if it e.g. parses a primary expression
    and sees it is followed by ++, --, ., ( or -> among other things it
    will try to reparse it as assignment expression or what and if that works
    suggests wrapping it inside of parens.
    When it is requires-clause that is after <typename T> etc. it already
    has an exception from that as ( can occur in valid C++20 expression there
    - starting the parameters of the lambda.
    In C++23 another case can occur, as the parameters with the ()s can be
    omitted, requires C can be followed immediately by -> which starts a
    trailing return type.  Even in that case, we don't want to parse that
    as C->...

    2021-04-16  Jakub Jelinek  <jakub@redhat.com>

            PR c++/99850
            * parser.c (cp_parser_constraint_requires_parens) <case CPP_DEREF>:
            If lambda_p, return pce_ok instead of pce_maybe_postfix.

            * g++.dg/cpp23/lambda-specifiers2.C: New test.

[Bug c++/99850] [P1102R2] reject valid lambda syntax in C++23

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99850

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
           Assignee|mpolacek at gcc dot gnu.org        |jakub at gcc dot gnu.org
             Status|ASSIGNED                    |RESOLVED

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed.