[Bug c++/96227] New: Comma operation sequencing issue

[Bug c++/96227] New: Comma operation sequencing issue

[matt at godbolt dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96227

            Bug ID: 96227
           Summary: Comma operation sequencing issue
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: matt at godbolt dot org
  Target Milestone: ---

Filing on behalf of @lunasorcery (on twitter):

The following code:

```
#include <cstdio>

struct S {
    S(char const* message) {
        puts(message);
    }
};
void operator,(S,S){}

int main() {
    S("first"), S("second");

    operator,(S("FIRST"), S("SECOND"));
}
```

Should produce the output:

first
second
FIRST
SECOND

But instead produces:

second
first
SECOND
FIRST


quotations from N4861:

[expr.comma]
"The left expression is sequenced before the right expression."

[over.match.oper]
"If either operand has a type that is a class or an enumeration,
a user-defined operator function might be declared that implements this
operator [...]
However, the operands are sequenced in the order prescribed for the built-in
operator."

[expr.call]
"If an operator function is invoked using operator notation, argument
evaluation is
sequenced as specified for the built-in operator; see 12.4.1.2."


https://gcc.godbolt.org/z/fdTG8q

[Bug c++/96227] Comma operation sequencing issue

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96227

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-07-16
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, Note this was partly fixed with the fix for PR 82959.
That is not invoked as a function case has been fixed for GCC 8.5.

On the trunk we produce as of today:
first
second
SECOND
FIRST

[Bug c++/96227] Comma operation sequencing issue

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96227

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |WAITING

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> Confirmed, Note this was partly fixed with the fix for PR 82959.
> That is not invoked as a function case has been fixed for GCC 8.5.
> 
> On the trunk we produce as of today:
> first
> second
> SECOND
> FIRST

I actually think this is still correct output as the function call version is
not mentioned in "If an operator function is invoked using operator notation"
here.

Am I reading this correctly?

[Bug c++/96227] Comma operation sequencing issue

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96227

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think so, the evaluation order when invoking operator, not through the x, y
syntax is like for any other function call, the arguments evaluation is
indeterminately sequenced.
See https://en.cppreference.com/w/cpp/language/eval_order

[Bug c++/96227] Comma operation sequencing issue

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96227

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |8.5
         Resolution|---                         |FIXED
             Status|WAITING                     |RESOLVED