[Boost-bugs] [Boost C++ Libraries] #11705: Phoenix function defining operator() with non-const reference parameter fails to compile with C++ 11.

Subject: [Boost-bugs] [Boost C++ Libraries] #11705: Phoenix function defining operator() with non-const reference parameter fails to compile with C++ 11.
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-10-03 05:07:56

#11705: Phoenix function defining operator() with non-const reference parameter
fails to compile with C++ 11.
-------------------------------------------------+---------------------
 Reporter:  David Williams <bateyware@…>         |      Owner:  theller
     Type:  Bugs                                 |     Status:  new
Milestone:  To Be Determined                     |  Component:  phoenix
  Version:  Boost 1.59.0                         |   Severity:  Problem
 Keywords:  phoenix function, C++ 11, result_of  |
-------------------------------------------------+---------------------
 A phoenix function object defining operator() with a non-const reference
 parameter does not compile when using clang version 3.5 in C++ 11 mode.

 The compiler used is: Apple LLVM version 6.0 (clang-600.0.57) (based on
 LLVM 3.5svn)

 The Boost version used is 1.59.

 A test program to illustrate the problem along with the compiler build log
 are provided with this bug report.

 An example of such a function object that reproduces the problem is the
 following:

 struct add_int_impl
 {
 typedef int result_type;

 add_int_impl(int v)
 {
         _value=v;
 }

 int operator()(bool& flag) const
 {
         std::cout << "int operator()(bool& flag) called" << std::endl;
         if (_value == 4)
                 flag=true;
         return _value;
 }

  int _value;
 };

 The function can be tested with the following code snippet:

 // *** Begin code snippet ***

 // Initialise the function with the value 4.

 boost::phoenix::function<add_int_impl> add_int(4);

 bool flag=false;

 BOOST_TEST(add_int(arg1)(flag) == (4));

 // *** End code snippet ***

 This call will only compile under Apple's version of clang 3.5 in C++ 11
 mode when one or both of the
 following conditions are met;

 1) The int operator()(int a) const is also defined

 2) One of the following flags is defined:

 BOOST_RESULT_OF_TR1
 BOOST_RESULT_OF_USE_TR1_WITH_DECLTYPE_FALLBACK

 The program also compiles if the function signature is changed to use a
 const
 reference parameter, i.e. int operator()(const bool& flag) const.
 However this then means that the parameter value cannot be
 modified within the body of the function as required.

 The problem does not occur if the program is compiled using C++ 98
 compatibility mode.

 I suspect that the problem relates to the use of boost::result_of, which
 uses the decltype() function
 to deduce function return types when compiling with C++ 11 support.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11705>
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