[Bug c++/102229] New: 'decltype(auto)' cannot be cv-qualified

[Bug c++/102229] New: 'decltype(auto)' cannot be cv-qualified

[netcan1996 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

            Bug ID: 102229
           Summary: 'decltype(auto)' cannot be cv-qualified
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: netcan1996 at gmail dot com
  Target Milestone: ---

code: https://godbolt.org/z/h833W9v95

it begins at version 11.x.

[Bug c++/102229] 'decltype(auto)' cannot be cv-qualified

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

Martin Liška <marxin at gcc dot gnu.org> changed:

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

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
struct Test {
    static constexpr decltype(auto) v = "test";
};
int main(int argc, char** argv) {

    return 0;
}

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |10.3.0
   Last reconfirmed|                            |2021-09-07
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Target Milestone|---                         |11.3
           Priority|P3                          |P2
      Known to fail|                            |11.1.0
           Keywords|                            |rejects-valid
            Summary|'decltype(auto)' cannot be  |[11/12 Regression]
                   |cv-qualified                |'decltype(auto)' cannot be
                   |                            |cv-qualified

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  clang accepts it as well.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|NEW                         |RESOLVED

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is desirable, added in my r11-2202 to fix bug 79815:

    "If the placeholder is the decltype(auto) type-specifier, T shall be the
    placeholder alone." but we weren't detecting "const decltype(auto)".

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |---
             Status|RESOLVED                    |NEW

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Maybe it's not desirable after all, it may be that type-specifiers are not
permitted but decl-specifiers are.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

Jason Merrill <jason at gcc dot gnu.org> changed:

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

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
The standard isn't very clear on this point; it seems like a core issue.

9.2.6/10: A constexpr specifier used in an object declaration declares the
object as const.

9.2.9.6.2/2.2: for a variable declared with a type that contains a placeholder
type, T is the declared type of the variable and E is the initializer.

/5: If the placeholder-type-specifier is of the form type-constraint opt
decltype(auto), T shall be the placeholder alone.

The question is whether constexpr adds const before deduction (so it's part of
"the declared type of the variable") or after.  The latter interpretation seems
more useful, so let's go with that.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Thanks.  With the latter interpretation in mind, it seems my earlier fix should
be mitigated so that we allow 

constexpr decltype(auto)
but not
constexpr const decltype(auto)

Yes?

Do you want me to raise this with the CWG?

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[netcan1996 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #7 from Net Can <netcan1996 at gmail dot com> ---
here is my use case:
https://github.com/netcan/config-loader/blob/master/include/config-loader/serialize/TypeSerializer.h

the key is
```c++
#define TYPE_SERIALIZER(_type, _typeName)                 \
    struct TypeSerializer<PARE _type>                     \
    { static constexpr decltype(auto) name = _typeName; }

template<> TYPE_SERIALIZER((int8_t), "int8_t"); // (1)

template<typename T>
TYPE_SERIALIZER((std::vector<T>),
                    concat("vector<", TypeSerializer<T>::name, ">")); // (2)
```

in case (1), the `name` type is `const char(&)[N]`, but in case(2), the `name`
type is `array[N]`, so I use `decltype(auto)` rather `auto`, becase latter
loses char length info.

I tried `static decltype(auto)` not work because it is defined in class, and
requires `constexpr`.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #6)
> Thanks.  With the latter interpretation in mind, it seems my earlier fix
> should be mitigated so that we allow 
> 
> constexpr decltype(auto)
> but not
> constexpr const decltype(auto)
> 
> Yes?

I think so, yes.

> Do you want me to raise this with the CWG?

Please.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

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++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #9 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This is supposed to be valid, so I'll try to come up with a fix soon.

[Bug c++/102229] [11/12 Regression] 'decltype(auto)' cannot be cv-qualified

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #10 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:06d5dcef72542baf49ac245cfde2ad7ecef0916b

commit r12-6008-g06d5dcef72542baf49ac245cfde2ad7ecef0916b
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Dec 10 15:38:35 2021 -0500

    c++: Allow constexpr decltype(auto) [PR102229]

    My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says
    "If the placeholder-type-specifier is of the form type-constraint[opt]
    decltype(auto), T shall be the placeholder alone."  But this made us
    reject 'constexpr decltype(auto)', which, after clarification from CWG,
    should be valid.  [dcl.type.auto.deduct]/4 is supposed to be a syntactic
    constraint, not semantic, so it's OK that the constexpr marks the object
    as const.

    As a consequence, checking TYPE_QUALS in do_auto_deduction is too late,
    and we have a FIXME there anyway.  So in this patch I'm attempting to
    detect 'const decltype(auto)' earlier.  If I'm going to use TYPE_QUALS,
    it needs to happen before we mark the object as const due to constexpr,
    that is, before grokdeclarator's

      /* A `constexpr' specifier used in an object declaration declares
         the object as `const'.  */
      if (constexpr_p && innermost_code != cdk_function)
        ...

    Constrained decltype(auto) was a little problem, hence the TYPENAME
    check.  But in a typename context you can't use decltype(auto) anyway,
    I think.

            PR c++/102229

    gcc/cp/ChangeLog:

            * decl.c (check_decltype_auto): New.
            (grokdeclarator): Call it.
            * pt.c (do_auto_deduction): Don't check decltype(auto) here.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/decltype-auto5.C: New test.

[Bug c++/102229] [11 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11/12 Regression]          |[11 Regression]
                   |'decltype(auto)' cannot be  |'decltype(auto)' cannot be
                   |cv-qualified                |cv-qualified

--- Comment #11 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.

[Bug c++/102229] [11 Regression] 'decltype(auto)' cannot be cv-qualified

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:24ee44c37a28eae80430a1ad86ffb0923446db2d

commit r11-9430-g24ee44c37a28eae80430a1ad86ffb0923446db2d
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Dec 10 15:38:35 2021 -0500

    c++: Allow constexpr decltype(auto) [PR102229]

    My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says
    "If the placeholder-type-specifier is of the form type-constraint[opt]
    decltype(auto), T shall be the placeholder alone."  But this made us
    reject 'constexpr decltype(auto)', which, after clarification from CWG,
    should be valid.  [dcl.type.auto.deduct]/4 is supposed to be a syntactic
    constraint, not semantic, so it's OK that the constexpr marks the object
    as const.

    As a consequence, checking TYPE_QUALS in do_auto_deduction is too late,
    and we have a FIXME there anyway.  So in this patch I'm attempting to
    detect 'const decltype(auto)' earlier.  If I'm going to use TYPE_QUALS,
    it needs to happen before we mark the object as const due to constexpr,
    that is, before grokdeclarator's

      /* A `constexpr' specifier used in an object declaration declares
         the object as `const'.  */
      if (constexpr_p && innermost_code != cdk_function)
        ...

    Constrained decltype(auto) was a little problem, hence the TYPENAME
    check.  But in a typename context you can't use decltype(auto) anyway,
    I think.

            PR c++/102229

    gcc/cp/ChangeLog:

            * decl.c (check_decltype_auto): New.
            (grokdeclarator): Call it.
            * pt.c (do_auto_deduction): Don't check decltype(auto) here.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/decltype-auto5.C: New test.

    (cherry picked from commit 06d5dcef72542baf49ac245cfde2ad7ecef0916b)

[Bug c++/102229] [11 Regression] 'decltype(auto)' cannot be cv-qualified

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102229

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

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

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