[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[mueller at gcc dot gnu dot org]

--- Comment #6 from mueller at gcc dot gnu dot org  2006-10-16 15:37 ---


*** This bug has been marked as a duplicate of 26167 ***


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mueller at gcc dot gnu dot org changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution||DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298

[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[rguenth at gcc dot gnu dot org]

--- Comment #5 from rguenth at gcc dot gnu dot org  2006-09-17 21:02 ---
Well, you are right.  This bug is a dup of the bug requesting -Wcoercion (if we
have such).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298

[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[rguenth at gcc dot gnu dot org]

--- Comment #3 from rguenth at gcc dot gnu dot org  2006-09-16 17:08 ---
26167, the one I added in the dependencies


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298

[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[lopezibanez at gmail dot com]

--- Comment #4 from lopezibanez at gmail dot com  2006-09-16 19:45 ---
Richard,

and what is your opinion about the rest of my comment?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298

[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[lopezibanez at gmail dot com]

--- Comment #2 from lopezibanez at gmail dot com  2006-09-16 01:38 ---
Richard, could you tell which bug report do you mean?

The -Wconversion warnings does not apply to this case. Wconversion warns about
the effect of adding a prototype, not about sign conversions. 

Anyway, I still don't understand where is the bug. Unary minus applied to
unsigned (what typically is the type of sizeof()) gives:

 `2^n - i', where `n' is the number of bits in the
unsigned type. [1]

So actually -sizeof(int) is 4294967292 (assuming sizeof(int) is 4). This number
fits well in a signed long long. However, it doesn't fit in a signed int, and
thus, it wraps around and produces -4.

The -Wcoercion flag provided by the Wcoercion project [2] warns for the
wrap-around as:

pr26298.c:15: warning: coercion as 'int' alters 'unsigned int' constant value


[1] http://gcc.gnu.org/ml/gcc/1999-06n/msg00754.html
[2] http://gcc.gnu.org/wiki/Wcoercion


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lopezibanez at gmail dot com changed:

   What|Removed |Added

 CC||lopezibanez at gmail dot
   ||com, rguenth at gcc dot gnu
   ||dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298

[Bug c++/26298] -Wconversion fails to detect signedness change during widening conversion

[rguenth at gcc dot gnu dot org]

--- Comment #1 from rguenth at gcc dot gnu dot org  2006-02-15 10:36 ---
With -Wconversion you get

t.c:5: warning: passing argument 1 of ‘toLongLong’ with different width due to
prototype

though -Wconversion is neither in -Wall nor -Wextra.  I see you filed against
C++, which doesn't warn here, but we have a bug for a similar case already.

Also the C diagnostic could be improved in the case of a widening sign
conversion,
because that is more likely an error than a non-widening sign conversion.


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rguenth at gcc dot gnu dot org changed:

   What|Removed |Added

  BugsThisDependsOn||26167
   Severity|normal  |enhancement
 Status|UNCONFIRMED |NEW
 Ever Confirmed|0   |1
   Keywords||diagnostic
   Last reconfirmed|-00-00 00:00:00 |2006-02-15 10:36:07
   date||
Summary|no warning for implicit type|-Wconversion fails to detect
   |conversion  |signedness change during
   ||widening conversion


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26298