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Bug#344501: marked as done (g++-4.0: spurious warning about explicit conversion operator to same type)

Your message dated Sun, 11 Apr 2010 14:30:33 +0000
with message-id <E1O0yBF-0008Jp-Qg@ravel.debian.org>
and subject line C++ and mudflap packages not built anymore from the gcc-4.1 source
has caused the Debian Bug report #344501,
regarding g++-4.0: spurious warning about explicit conversion operator to same type
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

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immediately.)


-- 
344501: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=344501
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
Package: g++-4.0
Version: 4.0.2-5
Severity: normal

Hi,

Given the following (stripped down) code as variant.cpp:

    #include <iostream>
    using std::cout;
    using std::endl;

    struct Variant
    {
        Variant() {}
        template< typename T > Variant( T ) {}
        Variant( const Variant& ) {}
        Variant &operator=( const Variant& ) { return *this; }

        template< typename T > operator T() const
        {
            cout << "operator T() called" << endl;
            return T();
        }
        template< typename T > operator const T&() const
        {   static T test_dummy;
            cout << "operator T&() called with T = Variant, uh oh" << endl;
            return test_dummy;
        }

      #ifdef WITH_WARNING
        operator Variant() const
        {
            cout << "operator Variant() called" << endl;
            return *this;
        }
        operator const Variant&() const
        {
            cout << "but operator Variant&() _is_ called" << endl;
            return *this;
        }
      #endif
    };

    Variant TempMaker( Variant v ) { return v; }

    int main()
    {
        Variant v = TempMaker( 1 );
        return 0;
    }

The following may be observed:

  $ make variant "CPPFLAGS=-Wconversion -DWITH_WARNING"
  g++  -Wconversion -DWITH_WARNING   variant.cpp   -o variant
  variant.cpp:39: warning: conversion to the same type will never use a
                  type conversion operator
  variant.cpp:44: warning: conversion to a reference to the same type
                  will never use a type conversion operator

  $ ./variant 
  but operator Variant&() _is_ called


  $ make variant CPPFLAGS=-Wconversion
  g++  -Wconversion   variant.cpp   -o variant

  $ ./variant 
  operator T&() called with T = Variant, uh oh


Indicating that the explicit operators are in fact called, despite
the warning, and are in fact necessary to avoid the compiler attempting
to use the template conversion operator with T = Variant(&) (and doing
bad recursive things in less stripped code) when temporaries are created
for the function call.

The simple answer to me is get rid of the warning, the code operates
intuitively correctly now, but things are rarely simple...

cheers,
Ron


-- System Information:
Debian Release: testing/unstable
  APT prefers unstable
  APT policy: (500, 'unstable')
Architecture: i386 (i686)
Shell:  /bin/sh linked to /bin/bash
Kernel: Linux 2.6.12
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968)

Versions of packages g++-4.0 depends on:
ii  gcc-4.0                       4.0.2-5    The GNU C compiler
ii  gcc-4.0-base                  4.0.2-5    The GNU Compiler Collection (base 
ii  libc6                         2.3.5-9    GNU C Library: Shared libraries an
ii  libstdc++6-4.0-dev            4.0.2-5    The GNU Standard C++ Library v3 (d

g++-4.0 recommends no packages.

-- no debconf information

--- End Message ---
--- Begin Message --- 
The C++ and mudflap binary packages are not built anymore from the
gcc-4.1 source package.  Tagging this report as won't fix, and closing
the report.

Please check if the problem is fixed in gcc-4.4 (or gcc-4.5/gcc-snapshot).
If the problem is not fixed, and not yet reported for the new compiler
version(s), open a new report.

--- End Message ---
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