[Bug c++/67499] c++ template/overload diagnostic compression

["manu at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67499

Manuel LÃpez-IbÃÃez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #5 from Manuel LÃpez-IbÃÃez <manu at gcc dot gnu.org> ---
There are several ways this output could be "easily" improved:

1) One problem with GCC's output is that it becomes difficult to track where
each candidate is listed in the wall of text. We could print something like
(also currently the candidates are not actually quoted, which is unusual when
printing source code):

note: candidate 1: %qD
...
note: candidate 2: %qD


2) Also, we print two times the same include stack for two different candidates
because we print a candidate from a different file in the middle. 

In file included from /usr/include/c++/4.8/ostream:612:0,
                 from /usr/include/c++/4.8/iostream:39,
                 from test.cc:1:
/usr/include/c++/4.8/bits/ostream.tcc:91:5: note: std::basic_ostream<_CharT,
_Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with
_CharT = char; _Traits = std::char_traits<char>]
     basic_ostream<_CharT, _Traits>::
     ^
/usr/include/c++/4.8/bits/ostream.tcc:91:5: note:   no known conversion for
argument 1 from âfooâ to âshort intâ
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from test.cc:1:
/usr/include/c++/4.8/ostream:181:7: note: std::basic_ostream<_CharT,
_Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short
unsigned int) [with _CharT = char; _Traits = std::char_traits<char>;
std::basic_ostream<_CharT, _Traits>::__ostr
eam_type = std::basic_ostream<char>]
       operator<<(unsigned short __n)
       ^
/usr/include/c++/4.8/ostream:181:7: note:   no known conversion for argument 1
from âfooâ to âshort unsigned intâ
In file included from /usr/include/c++/4.8/ostream:612:0,
                 from /usr/include/c++/4.8/iostream:39,
                 from test.cc:1:
/usr/include/c++/4.8/bits/ostream.tcc:105:5: note: std::basic_ostream<_CharT,
_Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT =
char; _Traits = std::char_traits<char>]
     basic_ostream<_CharT, _Traits>::
     ^

This is a lot of repetitive noise. The candidates could be sorted to reduce
this noise.


3) We print the reason in a different line than the candidate, with the caret
line in the middle. Thus, the caret line does not serve as a visual separation
between candidates.

/usr/include/c++/4.8/ostream:232:7: note: std::basic_ostream<_CharT,
_Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long
double) [with _CharT = char; _Traits = std::char_traits<char>;
std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(long double __f)
       ^
/usr/include/c++/4.8/ostream:232:7: note:   no known conversion for argument 1
from âfooâ to âlong doubleâ
/usr/include/c++/4.8/ostream:245:7: note: std::basic_ostream<_CharT,
_Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const
void*) [with _CharT = char; _Traits = std::char_traits<char>;
std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(const void* __p)
       ^
/usr/include/c++/4.8/ostream:245:7: note:   no known conversion for argument 1
from âfooâ to âconst void*â

Instead, we could print the reason after the candidate, perhaps even within the
same line. Maybe:

/usr/include/c++/4.8/ostream:232:7: note: candidate 5: no known conversion for
argument 1 from âfooâ to âlong doubleâ for 'std::basic_ostream<_CharT,
_Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long
double) [with _CharT = char; _Traits = std::char_traits<char>;
std::basic_ostream<_CharT, _Traits>::__ostream_type =
std::basic_ostream<char>]'
       operator<<(long double __f)
       ^
/usr/include/c++/4.8/ostream:245:7: note: candidate 6: no known conversion for
argument 1 from âfooâ to âconst void*â for 'std::basic_ostream<_CharT,
_Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const
void*) [with _CharT = char; _Traits = std::char_traits<char>;
std::basic_ostream<_CharT, _Traits>::__ostream_type =
std::basic_ostream<char>]'
       operator<<(const void* __p)
       ^

4) The candidate output is too verbose in some cases:

/usr/include/c++/4.8/ostream:548:5: note: template<class _Traits>
std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char,
_Traits>&, const unsigned char*)
     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
     ^
/usr/include/c++/4.8/ostream:548:5: note:   template argument
deduction/substitution failed:
test.cc:5:18: note:   cannot convert âbarâ (type âfooâ) to type âconst unsigned
char*â
     std::cout << bar;
                  ^

The above could simply say:

/usr/include/c++/4.8/ostream:548:5: note: template<class _Traits>
std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char,
_Traits>&, const unsigned char*)
     operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
     ^
/usr/include/c++/4.8/ostream:548:5: note:   template argument
deduction/substitution failed: cannot convert type âfooâ to type âconst
unsigned char*â

5) We could limit the number of candidates with a new option
(-fcandidates-limit=N) similar to -ftemplate-backtrace-limit=, with a
conservative value of 10 and printing a message "note: %d additional candidates
not shown, use -fcandidates-limit=0 to show all candidates"



Perhaps this could be Google Summer of Code project. Otherwise, I guess it will
not get implemented for lack of time/manpower.