[Bug c++/103826] New: Bogus shift-negative-value warning in C++20 mode

[Bug c++/103826] New: Bogus shift-negative-value warning in C++20 mode

[manx-bugzilla at problemloesungsmaschine dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103826

            Bug ID: 103826
           Summary: Bogus shift-negative-value warning in C++20 mode
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manx-bugzilla at problemloesungsmaschine dot de
  Target Milestone: ---

C++20 changed the semantics when shifting negative values from undefined
behavior to defined semantics. However, GCC still warns with -std=c++20 in this
case, which is unfortunate, because it generates a lot of warnings when porting
code to modern C++20. The warning is enabled by -Wextra.

```
int foo() {
    return (-32767) << 3;
}
```
`-std=c++20 -Wall -Wextra -Wpedantic`:
```
<source>: In function 'int foo()':
<source>:2:21: warning: left shift of negative value [-Wshift-negative-value]
    2 |     return (-32767) << 3;
      |            ~~~~~~~~~^~~~
```

Clang also warns in this case in C++17 mode, but does not warn in C++20 mode.
See <https://godbolt.org/z/8Yd9qfveP>.

[Bug c++/103826] Bogus shift-negative-value warning in C++20 mode

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103826

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-12-25
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, when the paper was implemented (r9-4132), this warning was missed it
seems.

Note I notice clang warns about:
int main() {
    return (0x7000'0000) << 4;
}

Which is valid now too, GCC disabled the warning for C++20 with r9-4132.

[Bug c++/103826] Bogus shift-negative-value warning in C++20 mode

[manx-bugzilla at problemloesungsmaschine dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103826

--- Comment #2 from Jörn Heusipp <manx-bugzilla at problemloesungsmaschine dot de> ---
(In reply to Andrew Pinski from comment #1)

> Note I notice clang warns about:
> int main() {
>     return (0x7000'0000) << 4;
> }

It's valid, but clang complains here about shifting bits out of the range of
the target type:
warning: signed shift result (0x700000000) requires 36 bits to represent, but
'int' only has 32 bits [-Wshift-overflow]

It does not warn when only shifting into the sign bit (and not overflowing the
bits of the target type):
return (0x7000'0000) << 1;

I am not sure what the intended semantics of -Wshift-overflow is for clang, but
I have reported <https://github.com/llvm/llvm-project/issues/52873>.