[LLVMbugs] [Bug 6898] New: Want better diagnostic for missing std::hash<T> specialization

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=6898

           Summary: Want better diagnostic for missing std::hash<T>
                    specialization
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Keywords: quality-of-implementation
          Severity: enhancement
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jyasskin at google.com
                CC: llvmbugs at cs.uiuc.edu, dgregor at apple.com


$ cat test.cc
#include <ext/hash_set>
#include <string>

void foo() {
    __gnu_cxx::hash_set<std::string> hs;
    hs.insert("Hello world");
}
$ clang++ -c test.cc
In file included from test.cc:1:
In file included from /usr/include/c++/4.0.0/ext/hash_set:64:
/usr/include/c++/4.0.0/ext/hashtable.h:596:16: error: type 'hasher const' (aka
'__gnu_cxx::hash<std::basic_string<char> > const') does not provide a call
operator
      { return _M_hash(__key) % __n; }
               ^~~~~~~
/usr/include/c++/4.0.0/ext/hashtable.h:600:16: note: in instantiation of member
function '__gnu_cxx::hashtable<std::basic_string<char>,
std::basic_string<char>, __gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >::_M_bkt_num_key' requested here
      { return _M_bkt_num_key(_M_get_key(__obj), __n); }
               ^
/usr/include/c++/4.0.0/ext/hashtable.h:1006:31: note: in instantiation of
member function '__gnu_cxx::hashtable<std::basic_string<char>,
std::basic_string<char>, __gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >::_M_bkt_num' requested here
                          size_type __new_bucket = _M_bkt_num(__first->_M_val,
                                                   ^
/usr/include/c++/4.0.0/ext/hashtable.h:438:2: note: in instantiation of member
function '__gnu_cxx::hashtable<std::basic_string<char>,
std::basic_string<char>, __gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >::resize' requested here
        resize(_M_num_elements + 1);
        ^
In file included from test.cc:1:
/usr/include/c++/4.0.0/ext/hash_set:206:43: note: in instantiation of member
function '__gnu_cxx::hashtable<std::basic_string<char>,
std::basic_string<char>, __gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >::insert_unique' requested here
        pair<typename _Ht::iterator, bool> __p = _M_ht.insert_unique(__obj);
                                                 ^
test.cc:6:5: note: in instantiation of member function
'__gnu_cxx::hash_set<std::basic_string<char>,
__gnu_cxx::hash<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
>
      >::insert' requested here
    hs.insert("Hello world");
    ^
1 error generated.



Thoughts on what we'd like from this:

* Ideally, we should say "Please provide a specialization for
'__gnu_cxx::hash<std::string>' that provides a 'size_t operator()(const
std::string&) const' member function". I'm not sure how to get this though.

* Simply removing the definition of "template<class _Key>struct hash" in
hash_fun.h produces a better message:

In file included from test.cc:1:
In file included from /usr/include/c++/4.0.0/ext/hash_set:64:
/usr/include/c++/4.0.0/ext/hashtable.h:304:29: error: implicit instantiation of
undefined template '__gnu_cxx::hash<std::basic_string<char> >'
      hasher                _M_hash;
                            ^
In file included from test.cc:1:
/usr/include/c++/4.0.0/ext/hash_set:102:11: note: in instantiation of template
class '__gnu_cxx::hashtable<std::basic_string<char>, std::basic_string<char>,
__gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >' requested here
      _Ht _M_ht;
          ^
test.cc:5:38: note: in instantiation of template class
'__gnu_cxx::hash_set<std::basic_string<char>,
__gnu_cxx::hash<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
>
      >' requested here
    __gnu_cxx::hash_set<std::string> hs;
                                     ^
In file included from test.cc:1:
In file included from /usr/include/c++/4.0.0/ext/hash_set:64:
In file included from /usr/include/c++/4.0.0/ext/hashtable.h:71:
/usr/include/c++/4.0.0/ext/hash_fun.h:71:12: note: template is declared here
    struct hash ;
           ^
1 error generated.


... but this requires changing libstdc++ and waiting for it to roll out.

* In the "in instantiation of template class", we should avoid re-printing the
"__gnu_cxx::hashtable<std::basic_string<char>, std::basic_string<char>,
__gnu_cxx::hash<std::basic_string<char> >,
      std::_Identity<std::basic_string<char> >,
std::equal_to<std::basic_string<char> >, std::allocator<std::basic_string<char>
> >" part. Perhaps we should replace it with "..." when it's the same as the
previous class name. Maybe say "in instantiation of member function from same
class '...::_M_bkt_num'.

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