[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