[Bug c++/113629] New: 'deducing this' does not work with conversion operators

[Bug c++/113629] New: 'deducing this' does not work with conversion operators

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629

            Bug ID: 113629
           Summary: 'deducing this' does not work with conversion
                    operators
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

It seems that GCC's deducing this implementation has some issues with derived
classes that inherit from a base class that has conversion operators:

struct Base {
  operator int(this auto&&) {
    return 42;
  }
};

struct Derived : Base {};

int main() {
  Derived derived;
  return static_cast<int>(derived);
}

GCC rejects the above code with:

<source>:11:10: error: invalid 'static_cast' from type 'Derived' to type 'int'
   11 |   return static_cast<int>(derived);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~

which seems wrong.

[Bug c++/113629] 'deducing this' does not work with conversion operators

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629

--- Comment #1 from 康桓瑋 <hewillk at gmail dot com> ---
test:
https://godbolt.org/z/jdz3ejohv

[Bug c++/113629] 'deducing this' does not work with conversion operators

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629

--- Comment #2 from 康桓瑋 <hewillk at gmail dot com> ---
more reduced:

struct Base {
  operator int(this auto&&) {
    return 42;
  }
};

int main() {
  Base b;
  // return static_cast<int>(Base{}); // ok
  return static_cast<int>(b);   // error
}

https://godbolt.org/z/qGrbf4rj7

[Bug c++/113629] 'deducing this' does not work with conversion operators

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathaniel Shead <nshead@gcc.gnu.org>:

https://gcc.gnu.org/g:49d83e963aa453600088380aebd507e172eb80ad

commit r14-9332-g49d83e963aa453600088380aebd507e172eb80ad
Author: Nathaniel Shead <nathanieloshead@gmail.com>
Date:   Wed Mar 6 00:43:22 2024 +1100

    c++: Fix template deduction for conversion operators with xobj parameters
[PR113629]

    Unification for conversion operators (DEDUCE_CONV) doesn't perform
    transformations like handling forwarding references. This is correct in
    general, but not for xobj parameters, which should be handled "normally"
    for the purposes of deduction: [temp.deduct.conv] only applies to the
    return type of the conversion function.

            PR c++/113629

    gcc/cp/ChangeLog:

            * pt.cc (type_unification_real): Only use DEDUCE_CONV for the
            return type of a conversion function.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp23/explicit-obj-conv-op.C: New test.

    Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

[Bug c++/113629] 'deducing this' does not work with conversion operators

[nshead at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113629

Nathaniel Shead <nshead at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.0
           Assignee|unassigned at gcc dot gnu.org      |nshead at gcc dot gnu.org
         Resolution|---                         |FIXED
                 CC|                            |nshead at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #4 from Nathaniel Shead <nshead at gcc dot gnu.org> ---
Fixed.