[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |84516

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
What is interesting is clang also reject this for the same reason.
Also this is related to PR 84516.

Then again this might be invalid based on [conv.prom] (I don't know if this
wording has changed in recent C++ versions though):


> A prvalue for an integral bit-field (10.3.10) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84516
[Bug 84516] bitfield temporaries > 32-bits have wrong type

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mital at mitalashok dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

Mital Ashok <mital at mitalashok dot co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mital at mitalashok dot co.uk

--- Comment #2 from Mital Ashok <mital at mitalashok dot co.uk> ---
This became CWG2627 https://cplusplus.github.io/CWG/issues/2627.html

Where the fix should mean that this shouldn't be narrowing:

```
void f(B b) {
   int{b.i};  // Not narrowing anymore
   b.i <=> b.i;  // Not narrowing anymore
   b <=> b;  // Not deleted anymore
}
```

GCC does not implement this yet, and I don't think this is related to PR 84516.

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-06-28
     Ever confirmed|0                           |1
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Thanks for the reference, I've updated
https://gcc.gnu.org/projects/cxx-dr-status.html

So confirmed.

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
This may implement that DR:

--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1012,6 +1012,14 @@ check_narrowing (tree type, tree init, tsubst_flags_t
complain,
       if (TREE_CODE (ftype) == ENUMERAL_TYPE)
    /* Check for narrowing based on the values of the enumeration. */
    ftype = ENUM_UNDERLYING_TYPE (ftype);
+      /* Undo convert_bitfield_to_declared_type.  (STRIP_NOPS would not be
+    enough).  */
+      tree stripped_init = init;
+      while (CONVERT_EXPR_P (stripped_init))
+   stripped_init = TREE_OPERAND (stripped_init, 0);
+      /* Core 2627  */
+      if (is_bitfield_expr_with_lowered_type (stripped_init))
+   ftype = TREE_TYPE (stripped_init);
       if ((tree_int_cst_lt (TYPE_MAX_VALUE (type),
                TYPE_MAX_VALUE (ftype))
       || tree_int_cst_lt (TYPE_MIN_VALUE (ftype),

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mital at mitalashok dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

--- Comment #5 from Mital Ashok <mital at mitalashok dot co.uk> ---
Proposed patch doesn't seem to work with bool bit-fields (with width not-1):

```
struct X {
  bool bf : 8;
} x;
signed char x_not_narrow{ x.bf };
```

Currently: Not narrowing
With patch:

```
test.cpp:4:29: warning: narrowing conversion of ‘(bool)x.X::bf’ from ‘unsigned
char’ to ‘signed char’ [-Wnarrowing]
    4 | signed char x_not_narrow{ x.bf };
      |                           ~~^~
```

This DR shouldn't affect this case since this is not a conversion "from an
integer type or unscoped enumeration type to an integer type that cannot
represent all the values of the original type" (bool -> signed char), so the
new exception shouldn't be checked.

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Right, I need to handle "the source is a bit-field whose width w is less than
that of its type" as well; is_bitfield_expr_with_lowered_type doesn't do that. 
Thanks!

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

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++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

--- Comment #7 from GCC 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:52d71b6b1f0f465a6cf064f61b22fc99453ec132

commit r15-1758-g52d71b6b1f0f465a6cf064f61b22fc99453ec132
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Jun 28 17:51:19 2024 -0400

    c++: DR2627, Bit-fields and narrowing conversions [PR94058]

    This DR (https://cplusplus.github.io/CWG/issues/2627.html) says that
    even if we are converting from an integer type or unscoped enumeration type
    to an integer type that cannot represent all the values of the original
    type, it's not narrowing if "the source is a bit-field whose width w is
    less than that of its type (or, for an enumeration type, its underlying
    type) and the target type can represent all the values of a hypothetical
    extended integer type with width w and with the same signedness as the
    original type".

            DR 2627
            PR c++/94058
            PR c++/104392

    gcc/cp/ChangeLog:

            * typeck2.cc (check_narrowing): Don't warn if the conversion isn't
            narrowing as per DR 2627.

    gcc/testsuite/ChangeLog:

            * g++.dg/DRs/dr2627.C: New test.
            * g++.dg/cpp0x/Wnarrowing22.C: New test.
            * g++.dg/cpp2a/spaceship-narrowing1.C: New test.
            * g++.dg/cpp2a/spaceship-narrowing2.C: New test.

[Bug c++/94058] defaulted three way comparison operator defined as deleted when a member is a small bitfield of long type.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94058

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

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

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Should be fixed in GCC 15.