<html>
    <head>
      <base href="https://llvm.org/bugs/" />
    </head>
    <body><table border="1" cellspacing="0" cellpadding="8">
        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW --- - map<const, const> fails copy-assignment"
   href="https://llvm.org/bugs/show_bug.cgi?id=23304">23304</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>map<const, const> fails copy-assignment
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>libc++
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>unspecified
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>PC
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Linux
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>All Bugs
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>unassignedclangbugs@nondot.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>eugeni.stepanov@gmail.com
          </td>
        </tr>

        <tr>
          <th>CC</th>
          <td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr></table>
      <p>
        <div>
        <pre>#include <string>
#include <map>

int main() {
  std::map<const std::string, const std::string> m1, m2;
  m1 = m2;
}

As I read the standard, the only requirement is that pair<const std::string,
const std::string> must be CopyInsertable, and it looks like it is.

Error text:

In file included from ../1.cc:1:
In file included from /code/llvm/build/bin/../include/c++/v1/string:439:
In file included from /code/llvm/build/bin/../include/c++/v1/algorithm:627:
/code/llvm/build/bin/../include/c++/v1/utility:299:15: error: no viable
overloaded '='
        first = __p.first;
        ~~~~~ ^ ~~~~~~~~~
/code/llvm/build/bin/../include/c++/v1/map:610:15: note: in instantiation of
member function 'std::__1::pair<const
      std::__1::basic_string<char>, const std::__1::basic_string<char>
<span class="quote">>::operator=' requested here</span >
        {__nc = __v.__cc; return *this;}
              ^
/code/llvm/build/bin/../include/c++/v1/__tree:1265:35: note: in instantiation
of member function
      'std::__1::__value_type<const std::__1::basic_string<char>, const
std::__1::basic_string<char> >::operator='
      requested here
                __cache->__value_ = *__first;
                                  ^
/code/llvm/build/bin/../include/c++/v1/__tree:1206:9: note: in instantiation of
function template specialization
      'std::__1::__tree<std::__1::__value_type<const
std::__1::basic_string<char>, const std::__1::basic_string<char> >,
      std::__1::__map_value_compare<const std::__1::basic_string<char>,
std::__1::__value_type<const
      std::__1::basic_string<char>, const std::__1::basic_string<char> >,
std::__1::less<const
      std::__1::basic_string<char> >, true>,
std::__1::allocator<std::__1::__value_type<const
      std::__1::basic_string<char>, const std::__1::basic_string<char> > >
     
<span class="quote">>::__assign_multi<std::__1::__tree_const_iterator<std::__1::__value_type<const</span >
std::__1::basic_string<char>, const
      std::__1::basic_string<char> >,
std::__1::__tree_node<std::__1::__value_type<const
std::__1::basic_string<char>,
      const std::__1::basic_string<char> >, void *> *, long> >' requested here
        __assign_multi(__t.begin(), __t.end());
        ^
/code/llvm/build/bin/../include/c++/v1/map:898:21: note: in instantiation of
member function
      'std::__1::__tree<std::__1::__value_type<const
std::__1::basic_string<char>, const std::__1::basic_string<char> >,
      std::__1::__map_value_compare<const std::__1::basic_string<char>,
std::__1::__value_type<const
      std::__1::basic_string<char>, const std::__1::basic_string<char> >,
std::__1::less<const
      std::__1::basic_string<char> >, true>,
std::__1::allocator<std::__1::__value_type<const
      std::__1::basic_string<char>, const std::__1::basic_string<char> > >
<span class="quote">>::operator=' requested here</span >
            __tree_ = __m.__tree_;
                    ^
../1.cc:6:6: note: in instantiation of member function 'std::__1::map<const
std::__1::basic_string<char>, const
      std::__1::basic_string<char>, std::__1::less<const
std::__1::basic_string<char> >,
      std::__1::allocator<std::__1::pair<const std::__1::basic_string<char>,
const std::__1::basic_string<char> > >
      >::operator=' requested here
  m1 = m2;</pre>
        </div>
      </p>
      <hr>
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