[Bug c++/104752] New: `Concept auto(x)` should not be a valid function-style cast

[Bug c++/104752] New: `Concept auto(x)` should not be a valid function-style cast

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104752

            Bug ID: 104752
           Summary: `Concept auto(x)` should not be a valid function-style
                    cast
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
                CC: mpolacek at gcc dot gnu.org
  Target Milestone: ---

https://godbolt.org/z/KbPMrbKsK

====
template<class T>
concept C = true;
auto x = auto(1);     // valid (P0849R8)
auto y = C auto(1);   // expect a error

====
The initializer of `y` should be an error, but is currently accepted by GCC.

[dcl.type.auto.deduct]:
> For an explicit type conversion ([expr.type.conv]), T is the specified type,
> which shall be auto.

Note that the specified type cannot be 'type-constraint auto'.

GCC does emit an error for `C<> auto(1)`.

[Bug c++/104752] `Concept auto(x)` should not be a valid function-style cast

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104752

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-02
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mine then.

[Bug c++/104752] `Concept auto(x)` should not be a valid function-style cast

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104752

--- Comment #2 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:97f76b5fc4b637033229e53033b4f8b6dc23472c

commit r12-7600-g97f76b5fc4b637033229e53033b4f8b6dc23472c
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Mar 2 12:12:33 2022 -0500

    c++: Don't allow type-constraint auto(x) [PR104752]

    104752 points out that

      template<class T>
      concept C = true;
      auto y = C auto(1);

    is ill-formed as per [dcl.type.auto.deduct]: "For an explicit type
conversion,
    T is the specified type, which shall be auto." which doesn't allow
    type-constraint auto.

            PR c++/104752

    gcc/cp/ChangeLog:

            * semantics.cc (finish_compound_literal): Disallow auto{x} for
            is_constrained_auto.
            * typeck2.cc (build_functional_cast_1): Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp23/auto-fncast12.C: New test.

[Bug c++/104752] `Concept auto(x)` should not be a valid function-style cast

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104752

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

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

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.