[Bug c++/108538] New: unexpected -Wnarrowing errors in -fpermissive mode

[Bug c++/108538] New: unexpected -Wnarrowing errors in -fpermissive mode

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108538

            Bug ID: 108538
           Summary: unexpected -Wnarrowing errors in -fpermissive mode
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stsp at users dot sourceforge.net
  Target Milestone: ---

int main()
{
    unsigned char a[1] = { -1 };
    return a[0];
}

$ g++ -fpermissive nar.cpp 
nar.cpp: In function ‘int main()’:
nar.cpp:3:28: error: narrowing conversion of ‘-1’ from ‘int’ to ‘unsigned char’
[-Wnarrowing]
    3 |     unsigned char a[1] = { -1 };


While I know that some -Wnarrowing
warnings were promoted to an errors,
was it the right decision also in
-fpermissive mode, which accepts most
of the C code?

[Bug c++/108538] unexpected -Wnarrowing errors in -fpermissive mode

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108538

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
It depends on the selected C++ standard.  C++11 does not allow narrowing
conversions unconditionally.

[Bug c++/108538] unexpected -Wnarrowing errors in -fpermissive mode

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108538

--- Comment #2 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Andreas Schwab from comment #1)
> It depends on the selected C++ standard.  C++11 does not allow narrowing
> conversions unconditionally.

Yes, I am not disputing that.
But I used -fpermissive mode to
compile the mix of c/c++.
-fpermissive downgrades many C++
errors to a warning, eating most
of the regular C. So my question
here is explicitly about -fpermissive
mode, I think it should downgrade
-Wnarrowing back into the warning.

[Bug c++/108538] unexpected -Wnarrowing errors in -fpermissive mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108538

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-01-25
             Status|UNCONFIRMED                 |NEW
           Severity|normal                      |enhancement
                 CC|                            |jason at gcc dot gnu.org
     Ever confirmed|0                           |1
           Keywords|                            |diagnostic

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Stas Sergeev from comment #2)
> But I used -fpermissive mode to
> compile the mix of c/c++.

It seems like you might be expecting more from -fpermissive than it actually
provides. It only affects a very limited set of diagnostics, and isn't a
general "compile invalid code" switch.

It might be reasonable to make it affect narrowing diagnostics though. The
downside would be complicating the code by adding even more interactions
between different switches and dialects.

Confirming as an enhancement request to relax some narrowing errors with
-fpermissive, but C++ front end maintainers should decide whether that's
actually desirable.

[Bug c++/108538] unexpected -Wnarrowing errors in -fpermissive mode

[stsp at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108538

--- Comment #4 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Jonathan Wakely from comment #3)
> It seems like you might be expecting more from -fpermissive than it actually
> provides. It only affects a very limited set of diagnostics, and isn't a
> general "compile invalid code" switch.

I always used it to compile the
(valid) C code in C++ mode. I thought
that's what it is for. It violates the
C++ standard up and down. And that
-Wnarrowing case is "better" than others
because it was a warning in c++03.
Other problems that -fpermissive allows,
were always an errors in any c++ mode.