[LLVMbugs] [Bug 16633] New: std::function return type cannot automatically cast to void

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16633

            Bug ID: 16633
           Summary: std::function return type cannot automatically cast to
                    void
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: hhinnant at apple.com
          Reporter: tatraian at deverto.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

I wrote the following c++ code:

#include <functional>

using namespace std;

bool fn(bool t){
    return t;
}


int main(int argc, char** argv){
    std::function<void ()> fns = std::bind(fn, true);
    return 0;
}


This code can be compiled with g++ 4.7 with libstdc++ and clang (trunk
-r186306) also with libstdc++. But I cannot compile with libc++ (trunk -r
186237) I got the next error message:

tatraian at server09:~/Pilots$ clang++ -std=c++11 -stdlib=libc++ -lc++abi
-nostdinc++ -I/home/tatraian/llvm-svn-libcxx/include/c++/v1 
-L/home/tatraian/llvm-svn-libcxx/lib/
-L/home/tatraian/tmp/llvm-svn-libcxxabi/lib/ bind.cc
bind.cc:11:28: error: no viable conversion from '__bind<bool (&)(bool), bool>'
to 'std::function<void ()>'
    std::function<void ()> fns = std::bind(fn, true);
                           ^     ~~~~~~~~~~~~~~~~~~~
/home/tatraian/llvm-svn-libcxx/include/c++/v1/functional:1138:5: note:
candidate constructor not viable: no known
      conversion from '__bind<bool (&)(bool), bool>' to 'nullptr_t' for 1st
argument
    function(nullptr_t) _NOEXCEPT : __f_(0) {}
    ^
/home/tatraian/llvm-svn-libcxx/include/c++/v1/functional:1139:5: note:
candidate constructor not viable: no known
      conversion from '__bind<bool (&)(bool), bool>' to 'const
std::__1::function<void ()> &' for 1st argument
    function(const function&);
    ^
/home/tatraian/llvm-svn-libcxx/include/c++/v1/functional:1140:5: note:
candidate constructor not viable: no known
      conversion from '__bind<bool (&)(bool), bool>' to
'std::__1::function<void ()> &&' for 1st argument
    function(function&&) _NOEXCEPT;
    ^
/home/tatraian/llvm-svn-libcxx/include/c++/v1/functional:1144:41: note:
candidate template ignored: disabled by
      'enable_if' [with _Fp = std::__1::__bind<bool (&)(bool), bool>]
                                        __callable<_Fp>::value &&
                                        ^ 

I think libc++' function takes care about return type. But I don't know if this
behaviour comes from C++ standard, or the standard says nothing about it or
just simply a bug.

In other way, it is logical that implicit cast does not work, but is there a
standard way to create a function object from an other function object that
differs only in return type (it changes return type to void)?

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