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

[manu at gcc dot gnu.org]

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

--- Comment #7 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #6)
> In the example above, you dropped the
> information that it is 'bar' (the second argument) that cannot be converted,
> which, for a function with more arguments and templates and typedefs all
> over the place, might not be obvious at all.

This info is already implied by the error:

test.cc:5:15: error: no match for ‘operator<<’ (operand types are ‘std::ostream
{aka std::basic_ostream<char>}’ and ‘foo’)
     std::cout << bar;
               ^

It is also inconsistently printed for some conversions:

/usr/include/c++/4.8/ostream:543:5: note: template<class _Traits>
std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char,
_Traits>&, const signed char*)
     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
     ^
/usr/include/c++/4.8/ostream:543:5: note:   template argument
deduction/substitution failed:
test.cc:5:18: note:   cannot convert ‘bar’ (type ‘foo’) to type ‘const signed
char*’
     std::cout << bar;
                  ^

but not for others:

/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; _Trai
ts = 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 ‘foo’ to ‘const void*’


Note that I'm ok with printing:

/usr/include/c++/4.8/ostream:543:5: note:   template argument
deduction/substitution failed: cannot convert ‘bar’ (type ‘foo’) to type ‘const
signed char*’

or even:

/usr/include/c++/4.8/ostream:543:5: note:   template argument
deduction/substitution failed: cannot convert ‘bar’ (type ‘foo’) to type ‘const
signed char*’ for argument 1

My point is to not print:

test.cc:5:18: note:  X
     std::cout << bar;
                  ^

for the error AND for every candidate. 

For example, we could print:

test.cc:5:15: error: no match for ‘operator<<’ (operand types are ‘std::ostream
{aka std::basic_ostream<char>}’ and ‘foo’)
     std::cout << bar;
               ^
test.cc:5:18: note: template argument deduction/substitution failed: cannot
convert ‘bar’ (type ‘foo’) 
     std::cout << bar;
                  ^
/usr/include/c++/4.8/ostream:245:7: note: candidate 1:  no known conversion for
argument 1 from ‘foo’ to ‘const void*’ 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)
       ^
/usr/include/c++/4.8/ostream:543:5: note: candidate 2: cannot convert type
‘foo’ to type ‘const signed char*’ for 'template<class _Traits>
std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char,
_Traits>&, const signed char*)'
     operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
     ^
[etc.]

> If we were outputting html, it would be much easier, we could print the
> error message and have javascript so that clicking in appropriate places
> displays the candidates, the reason for substitution failure, what a type is
> an alias for, etc. This is something IDEs could do. But with text, we have
> to print everything that might be useful.

We could still print it in a smarter, nicer looking and consistent way, without
repeating the same info several times (in the example above, there are only 108
unique lines out of 193).