[Bug c++/50224] New: [c++0x]g++ complains unused parameter but it is referenced in lambda

[lene13 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50224

             Bug #: 50224
           Summary: [c++0x]g++ complains unused parameter but it is
                    referenced in lambda
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: len...@gmail.com


#include <vector>
#include <algorithm>
#include <memory>

struct T;

struct S {
    void m(T& t);
};

struct C {
    std::vector<std::unique_ptr<S>> _s;

    void m(T& t) // ERROR here
    {
        std::for_each(_s.begin()
                    , _s.end()
                    , [&](std::unique_ptr<S> const& s)
                      {
                          s->m(t); // ``t`` is referenced here
                      });
    }
};

Compile that codes with

$ g++ test.cpp -c -Wunused-parameter -std=c++0x -Werror

and g++ will complain

test.cpp:14:10: error: unused parameter ‘t’ [-Werror=unused-parameter]

But if the parameter type changed to built in primitives, the error won't
occur, say, like

#include <vector>
#include <algorithm>
#include <memory>

struct S {
    void m(int t); // changed to int
};

struct C {
    std::vector<std::unique_ptr<S>> _s;

    void m(int t) // changed to int
    {
        std::for_each(_s.begin()
                    , _s.end()
                    , [&](std::unique_ptr<S> const& s)
                      {
                          s->m(t);
                      });
    }
};

My compiler and system info:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/src/gcc-4.6-20110819/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --enable-gnu-unique-object
--enable-linker-build-id --with-ppl --enable-cloog-backend=isl --enable-lto
--enable-gold --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold
--disable-multilib --disable-libssp --disable-libstdcxx-pch
--enable-checking=release
Thread model: posix
gcc version 4.6.1 20110819 (prerelease) (GCC) 

There is a similar bug I've reported before
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49339
marked as duplicated of
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49251
Hope that will help.