[Bug c++/40733] New: No warning is issued when an implicit conversion can lead to a data loss

[Bug c++/40733] New: No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

The following code contains an implicit integer conversion involving a data
loss. There is no warning issued even when -Wall is specified.

int main()
{
        int i = 0x12345678;
        char c = i;  // No warning is issued here
}


-- 
           Summary: No warning is issued when an implicit conversion can
                    lead to a data loss
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: photon at seznam dot cz


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2009-07-13 15:56 -------
-Wconversion needs to be supplied.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WORKSFORME


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

------- Comment #2 from photon at seznam dot cz  2009-07-13 20:57 -------
-Wall has a very misleading name and should probably be changed to match the
MSC behaviour (enable all warnings available).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2009-07-13 21:42 -------
(In reply to comment #2)
> -Wall has a very misleading name and should probably be changed to match the
> MSC behaviour (enable all warnings available).

"-Wall
This enables all the warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the warning),
even in conjunction with macros. "

hmm, I don't know a better name for -Wall really.  Since it enables all
warnings that are questionable and easy to avoid.  implicit integer conversion
is easy to avoid but is used all the place in normal C/C++ code. and with
integer promotion happening with simple stuff like a + b, some folks will have
a hard time to understand that happens which is why it is not enabled with
-Wall.

Plus there are style warnings not included in -Wall.  I really doubt you want
style warnings enabled with -Wall :).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

------- Comment #4 from photon at seznam dot cz  2009-07-14 08:37 -------
(In reply to comment #3)
> and with  integer promotion happening with simple stuff like a + b,
> some folks will have a hard time to understand that happens which
> is why it is not enabled with -Wall.

The warning is issued only for rare conversions involving a potential data loss
which should always be explicit.

> 
> Plus there are style warnings not included in -Wall.  I really doubt you want
> style warnings enabled with -Wall :).
> 

-Wall should be a real -Wall. Now it does not include important warnings like
those generated by -Wconversion (potential implicit data loss).

GCC should use numerical warning levels. The current -Wall should be renamed
to, e.g., -W3 and the new -Wall option should enable all warnings as the name
says.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[jakub at gcc dot gnu dot org]

------- Comment #5 from jakub at gcc dot gnu dot org  2009-07-14 09:24 -------
-Wconversion hits extremely often, it is definitely not a warning that can or
should be enabled in -Wall, nor in -W.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[manu at gcc dot gnu dot org]

------- Comment #6 from manu at gcc dot gnu dot org  2009-07-14 14:04 -------
There is a FAQ for the new Wconversion:

http://gcc.gnu.org/wiki/NewWconversion#faq

It should answer users' concerns.

As for Wall, we have users requesting less warnings from Wall and users
requesting more. We try to find a balance. But you are free to suggest that
existing warnings be moved to Wall or Wextra. However, first try compiling a
few big projects (like the kernel, QT libraries and KDE) and check that the
suggested warning only finds real bugs. Thanks.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

------- Comment #7 from photon at seznam dot cz  2009-07-14 15:13 -------
(In reply to comment #5)
> -Wconversion hits extremely often, it is definitely not a warning that can or
> should be enabled in -Wall, nor in -W.
> 

The fact that "it hits often" should not be an argument against including it in
the warning level -Wall. Implicit conversion to a smaller type occurs
infrequently and should always be explicit to avoid hard to find bugs.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

------- Comment #8 from photon at seznam dot cz  2009-07-14 15:31 -------
(In reply to comment #6)
> 
> As for Wall, we have users requesting less warnings from Wall and users
> requesting more. We try to find a balance. But you are free to suggest that
> existing warnings be moved to Wall or Wextra. 

There should be no discussion about adding or removing anything from the -Wall
level. It should include all warnings available. The current -Wall should be
renamed to something like -W3 to prevent misleading users. Each warning should
be assigned a warning level depending on the severity. The user should normally
only need to specify the desired warning level. -Wall should be used only
occasionally to make sure no warning is missed.


> However, first try compiling a
> few big projects (like the kernel, QT libraries and KDE) and check that the
> suggested warning only finds real bugs. Thanks.
> 

Are you suggesting that -Wconversion does not find real bugs?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2009-07-14 16:47 -------
(In reply to comment #8)
>  The current -Wall should be
> renamed to something like -W3 to prevent misleading users. 

It only misleading users who don't read the documentation.  The all part is
described in the documentation as:
"This enables all the warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the warning),
even in conjunction with macros. "

So the all is accurate in the sense it is all useful warnings; but it is not
all warnings.  

Most folks don't want -Wformat-nonliteral and -Wformat-security in their
warning set.  Or even -Wmissing-include-dirs or even -Winvalid-pch which will
warn if you have a PCH done for -O0 and one done for -O2 and you are compiling
at -O2 (and yes this happens all the time).

So your definition of -Wall is not very useful at all and will be even more
misleading to users or why the warnings are happening.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733

[Bug c++/40733] No warning is issued when an implicit conversion can lead to a data loss

[photon at seznam dot cz]

------- Comment #10 from photon at seznam dot cz  2009-07-14 18:11 -------
(In reply to comment #9)
> 
> So your definition of -Wall is not very useful at all and will be even more
> misleading to users or why the warnings are happening.
> 

MSC's /Wall enables all warnings and I don't find it misleading at all.

http://msdn.microsoft.com/en-us/library/thxezb7y(VS.71).aspx


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40733