[Bug c++/106111] New: -Wc++20-compat doesn't warn about using `requires` as an identifier

[Bug c++/106111] New: -Wc++20-compat doesn't warn about using `requires` as an identifier

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

            Bug ID: 106111
           Summary: -Wc++20-compat doesn't warn about using `requires` as
                    an identifier
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

int requires;

Clang warns about this:

r.C:1:5: warning: 'requires' is a keyword in C++20 [-Wc++20-compat]
int requires;
    ^
1 warning generated.

GCC should do too.

I wonder if we should warn about this by default (and suppress it if
-Wno-c++20-compat is used). A keyword that is a common English word (unlike
decltype, constexpr etc.) deserves a pretty big warning about compat with newer
standards.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Looks like we don't warn about any keywords with any -Wc++NN-compat options:

int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;

GCC doesn't warn about any of these, Clang only warns about two:

$ g++ -std=c++03 r.C -c -Wc++20-compat
$ clang++ -std=c++03 r.C -c -Wc++20-compat
r.C:1:26: warning: 'consteval' is a keyword in C++20 [-Wc++20-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                         ^
r.C:1:37: warning: 'requires' is a keyword in C++20 [-Wc++20-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                                    ^
2 warnings generated.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh, that's because -Wc++20-compat doesn't include -Wc++17-compat etc.

With the right options GCC warns about three:

$ g++ -std=c++03 r.C -c -Wc++20-compat -Wc++11-compat -Wc++14-compat
-Wc++17-compat
r.C:1:5: warning: identifier ‘decltype’ is a keyword in C++11 [-Wc++11-compat]
    1 | int decltype, constexpr, consteval, requires, noexcept, alignof,
alignas;
      |     ^~~~~~~~
r.C:1:15: warning: identifier ‘constexpr’ is a keyword in C++11
[-Wc++11-compat]
    1 | int decltype, constexpr, consteval, requires, noexcept, alignof,
alignas;
      |               ^~~~~~~~~
r.C:1:47: warning: identifier ‘noexcept’ is a keyword in C++11 [-Wc++11-compat]
    1 | int decltype, constexpr, consteval, requires, noexcept, alignof,
alignas;
      |                                               ^~~~~~~~

We're missing warnings for alignof and alignas in C++11, and consteval and
requires in C++20.

Clang warns about them all:

$ clang++ -std=c++03 r.C -c -Wc++20-compat -Wc++11-compat -Wc++14-compat
-Wc++17-compat
r.C:1:5: warning: 'decltype' is a keyword in C++11 [-Wc++11-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
    ^
r.C:1:15: warning: 'constexpr' is a keyword in C++11 [-Wc++11-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
              ^
r.C:1:26: warning: 'consteval' is a keyword in C++20 [-Wc++20-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                         ^
r.C:1:37: warning: 'requires' is a keyword in C++20 [-Wc++20-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                                    ^
r.C:1:47: warning: 'noexcept' is a keyword in C++11 [-Wc++11-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                                              ^
r.C:1:57: warning: 'alignof' is a keyword in C++11 [-Wc++11-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                                                        ^
r.C:1:66: warning: 'alignas' is a keyword in C++11 [-Wc++11-compat]
int decltype, constexpr, consteval, requires, noexcept, alignof, alignas;
                                                                 ^
7 warnings generated.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2022-06-28
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed, same problem with e.g.

int concept;

The problem is that cp_lexer_get_preprocessor_token has

  906           if (warn_cxx20_compat
  907               && C_RID_CODE (token->u.value) >= RID_FIRST_CXX20
  908               && C_RID_CODE (token->u.value) <= RID_LAST_CXX20)

but requires/concept aren't in the C++20 keywords, they are marked as
D_CXX_CONCEPTS_FLAGS instead.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
And for consteval the problem is likely that enum rid has:

 273   RID_FIRST_CXX20 = RID_CONSTINIT,
 274   RID_LAST_CXX20 = RID_CONSTINIT,

Oops.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

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

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

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:2ea6c59349793761b9c00f75ef281ac413566b2f

commit r13-1394-g2ea6c59349793761b9c00f75ef281ac413566b2f
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jun 28 18:59:19 2022 -0400

    c++: warn about using keywords as identifiers [PR106111]

    In C++03, -Wc++11-compat should warn about

      int constexpr;

    since 'constexpr' is a keyword in C++11.  Jonathan reports that
    we don't emit a similar warning for 'alignas' or 'alignof', and,
    as I found out, 'thread_local'.

    Similarly, we don't warn for most C++20 keywords.  That happens
    because RID_LAST_CXX20 hasn't been updated in a while.

            PR c++/106111

    gcc/c-family/ChangeLog:

            * c-common.h (enum rid): Update RID_LAST_CXX20.

    gcc/cp/ChangeLog:

            * parser.cc (cp_lexer_get_preprocessor_token): Also warn about
            RID_ALIGNOF, RID_ALIGNAS, RID_THREAD.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/keywords1.C: New test.
            * g++.dg/cpp2a/keywords1.C: New test.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed for GCC 13.

[Bug c++/106111] -Wc++20-compat doesn't warn about using `requires` as an identifier

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106111

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.0