[Bug c++/65322] New: Narrowing conversion

[Bug c++/65322] New: Narrowing conversion

[wolfgang.roehrl@gi-de.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

            Bug ID: 65322
           Summary: Narrowing conversion
           Product: gcc
           Version: 4.8.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wolfgang.roehrl@gi-de.com

Hi,
I would like to post a bug report for the GNU C/C++ compiler 4.8.3.
We use the compiler to generate code for a PowerPC processor.
Invokation line for the GNU C++ compiler:

ccppc -c -x c++ -std=c++11 -Wall -Werror -g -mcpu=8540 -meabi
      -ftls-model=local-exec -msdata=sysv -fno-common -mspe -mabi=spe
      -mfloat-gprs=double -mbig -mmultiple -mno-string -misel -mstrict-align
      -fverbose-asm -fno-exceptions -fno-rtti -fgcse-sm -fno-section-anchors
      -ftemplate-backtrace-limit=20 -G 8 -O3
      -I<some include paths>
      -D<some #define's>
      X.CPP -oX.O


// file X.CPP

#include "atomic"

struct S
{
    explicit S (bool b = true)
    : m_counter{ b ? 1u : 0u }
    {}

    std::atomic<unsigned short> m_counter;
};

S x;


The compiler rejects this programm with the following message:
x.CPP: In constructor 'S::S(bool)':
x.CPP:6:30: error: narrowing conversion of '(b ? 1u : 0u)' from 'unsigned int'
                   to 'std::atomic<short unsigned int>::__integral_type
                   {aka short unsigned int}' inside { } [-Werror=narrowing]
     : m_counter{ b ? 1u : 0u }


I think this is not standard conforming. See C++11 standard, 8.5.4/7:
"A narrowing conversion is an implicit conversion ... from an integer type or
unscoped enumeration type to an integer type that cannot represent all the
values of the original type, except where the source is a constant expression
and the actual value after conversion will fit into the target type and will
produce the original value when converted back to the original type."


Kind regards
W. Roehrl

[Bug c++/65322] Narrowing conversion

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

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

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
"b ? 1u : 0u" is not a constant expression, it depends on the variable b so the
value is not known until run-time.

[Bug c++/65322] Narrowing conversion

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. to make the code compile just don't use braces for the initialization:

    : m_counter( b ? 1u : 0u )

[Bug c++/65322] Narrowing conversion

[wolfgang.roehrl@gi-de.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

--- Comment #3 from Wolfgang Roehrl <wolfgang.roehrl@gi-de.com> ---
Hi Jonathan,

of course you are true. (I was overhasty with my bug report.)

Now I changed my program to using a constexpr-ctor:

#include "atomic"

struct S
{
    explicit constexpr S (bool b = true)
    : m_counter{ b ? 1u : 0u }
    {}

    std::atomic<unsigned short> m_counter;
};

S x;

The compiler rejects this program too - is this correct?

Wolfgang






Von:    "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
An:     wolfgang.roehrl@gi-de.com
Datum:  05.03.2015 11:54
Betreff:        [Bug c++/65322] Narrowing conversion



https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. to make the code compile just don't use braces for the 
initialization:

    : m_counter( b ? 1u : 0u )

[Bug c++/65322] Narrowing conversion

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #4)
> Just use m_counter( b ? 1u : 0u ) or m_counter{ short(b ? 1u : 0u) }

Oops, the second one should be m_counter{ (unsigned short)(b ? 1u : 0u) }

[Bug c++/65322] Narrowing conversion

[wolfgang.roehrl@gi-de.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65322

--- Comment #6 from Wolfgang Roehrl <wolfgang.roehrl@gi-de.com> ---
Thank you and have a nice day!


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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide319

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |ebotcazou at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |ebotcazou at gcc dot gnu.org