[Bug c++/84920] Better handling of unmatched/ambiguous calls

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.3                         |9.4

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 9.3.0 has been released, adjusting target milestone.

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |9.5

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9.4 is being released, retargeting bugs to GCC 9.5.

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[tdb at tdb dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

Mikko Rasa <tdb at tdb dot fi> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tdb at tdb dot fi

--- Comment #7 from Mikko Rasa <tdb at tdb dot fi> ---
Is this still being worked on?  I regularly run into spammy output, especially
with the == and != operators.  Just now I accidentally used std::find on an
std::map, which obviously didn't work - but spewed nearly 500 lines of notes
about all the operator== overloads it considered from several stdlib headers. 
At that point the notes cease to be useful and just make it harder to find out
where the actual error was.  This occurs with both g++-10 and g++-11.

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |---

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[lhyatt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

Lewis Hyatt <lhyatt at gcc dot gnu.org> changed:

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

--- Comment #8 from Lewis Hyatt <lhyatt at gcc dot gnu.org> ---
Created attachment 53108
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53108&action=edit
One possible approach

Hello-

What do you all think of this approach?

The new options -foverload-candidates-reduce-level and
-foverload-candidates-stop-level would configure the amount of information that
is output regarding overload candidates. When the number of overloads exceeds
-foverload-candidates-reduce-level (default 3), then the source lines are no
longer printed for each candidate, cutting the output length in half. The
source lines seem rarely useful anyway, since the diagnostic's first line
already prints the overload being considered. When the number of overloads
exceeds -foverload-candidates-stop-level (default 7), none of the overloads is
printed, since this is presumably a case such as a heavily overloaded operator
where the large number of candidates is less likely to be helpful in fixing the
problem.

For a testcase like this common (for me) typo:

=============
#include <ostream>
void f(std::ostream& o, int a) {
    o << a < '\n';
}
=============

GCC then outputs:
=============
iowarn.cpp: In function ‘void f(std::ostream&, int)’:
iowarn.cpp:3:12: error: no match for ‘operator<’ (operand types are
‘std::basic_ostream<char>’ and ‘char’)
    3 |     o << a < '\n';
      |     ~~~~~~ ^ ~~~~
      |       |      |
      |       |      char
      |       std::basic_ostream<char>
iowarn.cpp:3:12: note: declining to output 15 candidates (limit 7); compile
with ‘-foverload-candidates-stop-level=15’ to see them
=============

which at least for this specific case feels like an improvement to me... It's
obvious what is wrong now.

If you follow the suggestion and add the flag to the compile, then you see:
=============
iowarn.cpp: In function ‘void f(std::ostream&, int)’:
iowarn.cpp:3:12: error: no match for ‘operator<’ (operand types are
‘std::basic_ostream<char>’ and ‘char’)
    3 |     o << a < '\n';
      |     ~~~~~~ ^ ~~~~
      |       |      |
      |       |      char
      |       std::basic_ostream<char>
iowarn.cpp:3:12: note: candidate: ‘operator<(int, int)’ (built-in)
iowarn.cpp:3:12: note:   no known conversion for argument 1 from
‘std::basic_ostream<char>’ to ‘int’
In file included from /home/lewis/gccdev/release/include/c++/13.0.0/string:46,
                 from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/locale_classes.h:40,
                 from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/ios_base.h:41,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/ios:42,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/ostream:38,
                 from iowarn.cpp:1:
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:451:5: note:
candidate: ‘template<class _Iterator> constexpr bool std::operator<(const
reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:451:5: note: 
 template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::reverse_iterator<_Iterator>’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:496:5: note:
candidate: ‘template<class _IteratorL, class _IteratorR> constexpr bool
std::operator<(const reverse_iterator<_Iterator>&, const
reverse_iterator<_IteratorR>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:496:5: note: 
 template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::reverse_iterator<_Iterator>’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1683:5: note:
candidate: ‘template<class _IteratorL, class _IteratorR> constexpr bool
std::operator<(const move_iterator<_IteratorL>&, const
move_iterator<_IteratorR>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1683:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::move_iterator<_IteratorL>’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1748:5: note:
candidate: ‘template<class _Iterator> constexpr bool std::operator<(const
move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1748:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::move_iterator<_IteratorL>’
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_algobase.h:64,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:49:
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_pair.h:665:5: note:
candidate: ‘template<class _T1, class _T2> constexpr bool std::operator<(const
pair<_T1, _T2>&, const pair<_T1, _T2>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_pair.h:665:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::pair<_T1, _T2>’
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:47,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:52:
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:594:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT,
_Traits>)’
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:594:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:600:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(basic_string_view<_CharT, _Traits>,
__type_identity_t<basic_string_view<_CharT, _Traits> >)’
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:600:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:607:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >,
basic_string_view<_CharT, _Traits>)’
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:607:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   mismatched types ‘std::basic_string_view<_CharT,
_Traits>’ and ‘char’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3690:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const
__cxx11::basic_string<_CharT, _Traits, _Allocator>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3690:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3703:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const
_CharT*)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3703:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3715:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits,
_Allocator>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3715:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   mismatched types ‘const _CharT*’ and
‘std::basic_ostream<char>’
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/memory_resource.h:47,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:56:
/home/lewis/gccdev/release/include/c++/13.0.0/tuple:1538:5: note: candidate:
‘template<class ... _TElements, class ... _UElements> constexpr bool
std::operator<(const tuple<_UTypes ...>&, const tuple<_UTypes ...>&)’
/home/lewis/gccdev/release/include/c++/13.0.0/tuple:1538:5: note:   template
argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::tuple<_UTypes ...>’
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/ios_base.h:46:
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:314:3: note:
candidate: ‘bool std::operator<(const error_code&, const error_code&)’
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:314:31: note:   no
known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const
std::error_code&’
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:495:3: note:
candidate: ‘bool std::operator<(const error_condition&, const
error_condition&)’
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:495:36: note:   no
known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const
std::error_condition&’
=============

This is still half the length of the current output, because the source lines
are not printed. The current output is:
=============
iowarn.cpp: In function ‘void f(std::ostream&, int)’:
iowarn.cpp:3:12: error: no match for ‘operator<’ (operand types are
‘std::basic_ostream<char>’ and ‘char’)
    3 |     o << a < '\n';
      |     ~~~~~~ ^ ~~~~
      |       |      |
      |       |      char
      |       std::basic_ostream<char>
iowarn.cpp:3:12: note: candidate: ‘operator<(int, int)’ (built-in)
    3 |     o << a < '\n';
      |     ~~~~~~~^~~~~~
iowarn.cpp:3:12: note:   no known conversion for argument 1 from
‘std::basic_ostream<char>’ to ‘int’
In file included from /home/lewis/gccdev/release/include/c++/13.0.0/string:46,
                 from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/locale_classes.h:40,
                 from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/ios_base.h:41,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/ios:42,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/ostream:38,
                 from iowarn.cpp:1:
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:451:5: note:
candidate: ‘template<class _Iterator> constexpr bool std::operator<(const
reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)’
  451 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:451:5: note: 
 template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::reverse_iterator<_Iterator>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:496:5: note:
candidate: ‘template<class _IteratorL, class _IteratorR> constexpr bool
std::operator<(const reverse_iterator<_Iterator>&, const
reverse_iterator<_IteratorR>&)’
  496 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:496:5: note: 
 template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::reverse_iterator<_Iterator>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1683:5: note:
candidate: ‘template<class _IteratorL, class _IteratorR> constexpr bool
std::operator<(const move_iterator<_IteratorL>&, const
move_iterator<_IteratorR>&)’
 1683 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1683:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::move_iterator<_IteratorL>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1748:5: note:
candidate: ‘template<class _Iterator> constexpr bool std::operator<(const
move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)’
 1748 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_iterator.h:1748:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::move_iterator<_IteratorL>’
    3 |     o << a < '\n';
      |              ^~~~
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_algobase.h:64,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:49:
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_pair.h:665:5: note:
candidate: ‘template<class _T1, class _T2> constexpr bool std::operator<(const
pair<_T1, _T2>&, const pair<_T1, _T2>&)’
  665 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/stl_pair.h:665:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::pair<_T1, _T2>’
    3 |     o << a < '\n';
      |              ^~~~
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:47,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:52:
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:594:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT,
_Traits>)’
  594 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:594:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:600:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(basic_string_view<_CharT, _Traits>,
__type_identity_t<basic_string_view<_CharT, _Traits> >)’
  600 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:600:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:607:5: note:
candidate: ‘template<class _CharT, class _Traits> constexpr bool
std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >,
basic_string_view<_CharT, _Traits>)’
  607 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>>
__x,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/string_view:607:5: note:  
template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   mismatched types ‘std::basic_string_view<_CharT,
_Traits>’ and ‘char’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3690:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const
__cxx11::basic_string<_CharT, _Traits, _Allocator>&)’
 3690 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3690:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3703:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const
_CharT*)’
 3703 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3703:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
    3 |     o << a < '\n';
      |              ^~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3715:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc> bool
std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits,
_Allocator>&)’
 3715 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/bits/basic_string.h:3715:5: note:
  template argument deduction/substitution failed:
iowarn.cpp:3:14: note:   mismatched types ‘const _CharT*’ and
‘std::basic_ostream<char>’
    3 |     o << a < '\n';
      |              ^~~~
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/memory_resource.h:47,
                 from /home/lewis/gccdev/release/include/c++/13.0.0/string:56:
/home/lewis/gccdev/release/include/c++/13.0.0/tuple:1538:5: note: candidate:
‘template<class ... _TElements, class ... _UElements> constexpr bool
std::operator<(const tuple<_UTypes ...>&, const tuple<_UTypes ...>&)’
 1538 |     operator<(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/tuple:1538:5: note:   template
argument deduction/substitution failed:
iowarn.cpp:3:14: note:   ‘std::basic_ostream<char>’ is not derived from ‘const
std::tuple<_UTypes ...>’
    3 |     o << a < '\n';
      |              ^~~~
In file included from
/home/lewis/gccdev/release/include/c++/13.0.0/bits/ios_base.h:46:
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:314:3: note:
candidate: ‘bool std::operator<(const error_code&, const error_code&)’
  314 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |   ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:314:31: note:   no
known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const
std::error_code&’
  314 |   operator<(const error_code& __lhs, const error_code& __rhs) noexcept
      |             ~~~~~~~~~~~~~~~~~~^~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:495:3: note:
candidate: ‘bool std::operator<(const error_condition&, const
error_condition&)’
  495 |   operator<(const error_condition& __lhs,
      |   ^~~~~~~~
/home/lewis/gccdev/release/include/c++/13.0.0/system_error:495:36: note:   no
known conversion for argument 1 from ‘std::basic_ostream<char>’ to ‘const
std::error_condition&’
  495 |   operator<(const error_condition& __lhs,
      |             ~~~~~~~~~~~~~~~~~~~~~~~^~~~~
=============

Perhaps the default thresholds of 3 and 7 could be tuned better based on
experience, but I hope maybe it's at least a useful framework to set up? If so
then the patch is ready to go and I could send it to gcc-patches. Thanks for
taking a look!

-Lewis

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That looks pretty good to me. What does it produce for the operator<< example
in comment 1?

[Bug c++/84920] Better handling of unmatched/ambiguous calls

[lhyatt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84920

--- Comment #10 from Lewis Hyatt <lhyatt at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #9)
> That looks pretty good to me. What does it produce for the operator<<
> example in comment 1?

With the default options:
=============
iowarn2.cpp: In function ‘void f(std::ostream&, const A<Y>&)’:
iowarn2.cpp:13:5: error: no match for ‘operator<<’ (operand types are
‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘const A<Y>’)
   13 |   o << a;
      |   ~ ^~ ~
      |   |    |
      |   |    const A<Y>
      |   std::ostream {aka std::basic_ostream<char>}
iowarn2.cpp:13:5: note: declining to output 34 candidates (limit 7); compile
with ‘-foverload-candidates-stop-level=34’ to see them
=============

With the suggestion from the diagnostic -foverload-candidates-stop-level=34, so
this is the ~50% abbreviated mode:
=============
iowarn2.cpp: In function ‘void f(std::ostream&, const A<Y>&)’:
iowarn2.cpp:13:5: error: no match for ‘operator<<’ (operand types are
‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘const A<Y>’)
   13 |   o << a;
      |   ~ ^~ ~
      |   |    |
      |   |    const A<Y>
      |   std::ostream {aka std::basic_ostream<char>}
In file included from iowarn2.cpp:1:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:108:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(__ostream_type&
(*)(__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>;
__ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:108:36: note:   no known
conversion for argument 1 from ‘const A<Y>’ to
‘std::basic_ostream<char>::__ostream_type&
(*)(std::basic_ostream<char>::__ostream_type&)’ {aka ‘std::basic_ostream<char>&
(*)(std::basic_ostream<char>&)’}
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:117:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(__ios_type& (*)(__ios_type&))
[with _CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>; __ios_type = std::basic_ios<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:117:32: note:   no known
conversion for argument 1 from ‘const A<Y>’ to
‘std::basic_ostream<char>::__ios_type&
(*)(std::basic_ostream<char>::__ios_type&)’ {aka ‘std::basic_ios<char>&
(*)(std::basic_ios<char>&)’}
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:127:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base&
(*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>;
__ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:127:30: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘std::ios_base&
(*)(std::ios_base&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:166:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:166:23: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘long int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:170:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT
= char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:170:32: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘long unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:174:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:174:23: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘bool’
In file included from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:833:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:91:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(short int) [with _CharT = char; _Traits =
std::char_traits<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:92:22: note:  
no known conversion for argument 1 from ‘const A<Y>’ to ‘short int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:181:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with
_CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:181:33: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘short unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:105:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(int) [with _CharT = char; _Traits =
std::char_traits<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:106:20: note:  
no known conversion for argument 1 from ‘const A<Y>’ to ‘int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:192:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:192:31: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:201:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:201:28: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘long long int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:205:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with
_CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:205:37: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘long long unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:220:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:220:25: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘double’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:224:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:224:24: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘float’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:232:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:232:30: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘long double’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:245:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:245:30: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘const void*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:250:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(std::nullptr_t) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>; std::nullptr_t = std::nullptr_t]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:250:18: note:   no known
conversion for argument 1 from ‘const A<Y>’ to ‘std::nullptr_t’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:119:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(__streambuf_type*) [with _CharT = char; _Traits =
std::char_traits<char>; __streambuf_type = std::basic_streambuf<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:120:34: note:  
no known conversion for argument 1 from ‘const A<Y>’ to
‘std::basic_ostream<char>::__streambuf_type*’ {aka
‘std::basic_streambuf<char>*’}
iowarn2.cpp:5:6: note: candidate: ‘template<class T> decltype ((o << a.value))
operator<<(std::ostream&, const A<T>&)’
iowarn2.cpp:5:6: note:   template argument deduction/substitution failed:
iowarn2.cpp: In substitution of ‘template<class T> decltype ((o << a.value))
operator<<(std::ostream&, const A<T>&) [with T = Y]’:
iowarn2.cpp:13:8:   required from here
iowarn2.cpp:6:15: error: no match for ‘operator<<’ (operand types are
‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘const Y’)
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:108:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(__ostream_type&
(*)(__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>;
__ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:108:36: note:   no known
conversion for argument 1 from ‘const Y’ to
‘std::basic_ostream<char>::__ostream_type&
(*)(std::basic_ostream<char>::__ostream_type&)’ {aka ‘std::basic_ostream<char>&
(*)(std::basic_ostream<char>&)’}
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:117:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(__ios_type& (*)(__ios_type&))
[with _CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>; __ios_type = std::basic_ios<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:117:32: note:   no known
conversion for argument 1 from ‘const Y’ to
‘std::basic_ostream<char>::__ios_type&
(*)(std::basic_ostream<char>::__ios_type&)’ {aka ‘std::basic_ios<char>&
(*)(std::basic_ios<char>&)’}
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:127:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base&
(*)(std::ios_base&)) [with _CharT = char; _Traits = std::char_traits<char>;
__ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:127:30: note:   no known
conversion for argument 1 from ‘const Y’ to ‘std::ios_base&
(*)(std::ios_base&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:166:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:166:23: note:   no known
conversion for argument 1 from ‘const Y’ to ‘long int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:170:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT
= char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:170:32: note:   no known
conversion for argument 1 from ‘const Y’ to ‘long unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:174:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:174:23: note:   no known
conversion for argument 1 from ‘const Y’ to ‘bool’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:91:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(short int) [with _CharT = char; _Traits =
std::char_traits<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:92:22: note:  
no known conversion for argument 1 from ‘const Y’ to ‘short int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:181:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with
_CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:181:33: note:   no known
conversion for argument 1 from ‘const Y’ to ‘short unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:105:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(int) [with _CharT = char; _Traits =
std::char_traits<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:106:20: note:  
no known conversion for argument 1 from ‘const Y’ to ‘int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:192:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:192:31: note:   no known
conversion for argument 1 from ‘const Y’ to ‘unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:201:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:201:28: note:   no known
conversion for argument 1 from ‘const Y’ to ‘long long int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:205:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with
_CharT = char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:205:37: note:   no known
conversion for argument 1 from ‘const Y’ to ‘long long unsigned int’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:220:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:220:25: note:   no known
conversion for argument 1 from ‘const Y’ to ‘double’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:224:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char;
_Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:224:24: note:   no known
conversion for argument 1 from ‘const Y’ to ‘float’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:232:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:232:30: note:   no known
conversion for argument 1 from ‘const Y’ to ‘long double’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:245:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:245:30: note:   no known
conversion for argument 1 from ‘const Y’ to ‘const void*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:250:7: note: candidate:
‘std::basic_ostream<_CharT, _Traits>::__ostream_type&
std::basic_ostream<_CharT, _Traits>::operator<<(std::nullptr_t) [with _CharT =
char; _Traits = std::char_traits<char>; __ostream_type =
std::basic_ostream<char>; std::nullptr_t = std::nullptr_t]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:250:18: note:   no known
conversion for argument 1 from ‘const Y’ to ‘std::nullptr_t’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:119:5: note:
candidate: ‘std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT,
_Traits>::operator<<(__streambuf_type*) [with _CharT = char; _Traits =
std::char_traits<char>; __streambuf_type = std::basic_streambuf<char>]’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:120:34: note:  
no known conversion for argument 1 from ‘const Y’ to
‘std::basic_ostream<char>::__streambuf_type*’ {aka
‘std::basic_streambuf<char>*’}
In file included from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/basic_string.h:47,
                 from /home/lewis/gccdev/dev/rel/include/c++/13.0.0/string:52,
                 from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/locale_classes.h:40,
                 from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ios_base.h:41,
                 from /home/lewis/gccdev/dev/rel/include/c++/13.0.0/ios:42,
                 from /home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:38:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/string_view:672:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&,
basic_string_view<_CharT, _Traits>)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/string_view:672:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   ‘Y’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/basic_string.h:3882:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc>
std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT,
_Traits>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/basic_string.h:3882:5: note:
  template argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   ‘const Y’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
In file included from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/memory_resource.h:38,
                 from /home/lewis/gccdev/dev/rel/include/c++/13.0.0/string:56:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/cstddef:123:5: note: candidate:
‘template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType>
std::operator<<(byte, _IntegerType)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/cstddef:123:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   cannot convert ‘o’ (type ‘std::ostream’ {aka
‘std::basic_ostream<char>’}) to type ‘std::byte’
In file included from
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ios_base.h:46:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/system_error:329:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const error_code&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/system_error:329:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘const std::error_code&’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:507:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, _CharT)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:507:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   deduced conflicting types for parameter ‘_CharT’
(‘char’ and ‘Y’)
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:517:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:517:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:523:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:523:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:534:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, signed char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:534:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘signed char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:539:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, unsigned char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:539:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘unsigned char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:598:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, const _CharT*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:598:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   mismatched types ‘const _CharT*’ and ‘Y’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:302:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:302:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘const char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:615:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:615:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘const char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:628:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const signed char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:628:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘const signed char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:633:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const unsigned char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:633:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:6:20: note:   cannot convert ‘a.A<Y>::value’ (type ‘const Y’) to
type ‘const unsigned char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: note: candidate:
‘template<class _Ostream, class _Tp> _Ostream&& std::operator<<(_Ostream&&,
const _Tp&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: note:   template
argument deduction/substitution failed:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream: In substitution of
‘template<class _Ostream, class _Tp> _Ostream&& std::operator<<(_Ostream&&,
const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = Y]’:
iowarn2.cpp:6:15:   required by substitution of ‘template<class T> decltype ((o
<< a.value)) operator<<(std::ostream&, const A<T>&) [with T = Y]’
iowarn2.cpp:13:8:   required from here
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: error: no type
named ‘type’ in ‘struct std::enable_if<false, void>’
iowarn2.cpp: In substitution of ‘template<class T> decltype ((o << a.value))
operator<<(std::ostream&, const A<T>&) [with T = Y]’:
iowarn2.cpp:13:8:   required from here
iowarn2.cpp:5:6: note: candidate: ‘template<class T> decltype ((o << a.value))
operator<<(std::ostream&, const A<T>&)’
iowarn2.cpp:5:6: note:   template argument deduction/substitution failed:
iowarn2.cpp:6:15: note:   ‘const Y’ is not derived from ‘const A<T>’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/string_view:672:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&,
basic_string_view<_CharT, _Traits>)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/string_view:672:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   ‘A<Y>’ is not derived from
‘std::basic_string_view<_CharT, _Traits>’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/basic_string.h:3882:5: note:
candidate: ‘template<class _CharT, class _Traits, class _Alloc>
std::basic_ostream<_CharT, _Traits>& std::operator<<(basic_ostream<_CharT,
_Traits>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/basic_string.h:3882:5: note:
  template argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   ‘const A<Y>’ is not derived from ‘const
std::__cxx11::basic_string<_CharT, _Traits, _Allocator>’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/cstddef:123:5: note: candidate:
‘template<class _IntegerType> constexpr std::__byte_op_t<_IntegerType>
std::operator<<(byte, _IntegerType)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/cstddef:123:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:3: note:   cannot convert ‘o’ (type ‘std::ostream’ {aka
‘std::basic_ostream<char>’}) to type ‘std::byte’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/system_error:329:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const error_code&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/system_error:329:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘const
std::error_code&’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:507:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, _CharT)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:507:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   deduced conflicting types for parameter ‘_CharT’
(‘char’ and ‘A<Y>’)
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:517:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:517:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:523:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:523:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:534:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, signed char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:534:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type
‘signed char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:539:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, unsigned char)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:539:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type
‘unsigned char’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:598:5: note: candidate:
‘template<class _CharT, class _Traits> std::basic_ostream<_CharT, _Traits>&
std::operator<<(basic_ostream<_CharT, _Traits>&, const _CharT*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:598:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   mismatched types ‘const _CharT*’ and ‘A<Y>’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:302:5: note:
candidate: ‘template<class _CharT, class _Traits> std::basic_ostream<_CharT,
_Traits>& std::operator<<(basic_ostream<_CharT, _Traits>&, const char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/bits/ostream.tcc:302:5: note:  
template argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘const
char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:615:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:615:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘const
char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:628:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const signed char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:628:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘const
signed char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:633:5: note: candidate:
‘template<class _Traits> std::basic_ostream<char, _Traits>&
std::operator<<(basic_ostream<char, _Traits>&, const unsigned char*)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:633:5: note:   template
argument deduction/substitution failed:
iowarn2.cpp:13:8: note:   cannot convert ‘a’ (type ‘const A<Y>’) to type ‘const
unsigned char*’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: note: candidate:
‘template<class _Ostream, class _Tp> _Ostream&& std::operator<<(_Ostream&&,
const _Tp&)’
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: note:   template
argument deduction/substitution failed:
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream: In substitution of
‘template<class _Ostream, class _Tp> _Ostream&& std::operator<<(_Ostream&&,
const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = A<Y>]’:
iowarn2.cpp:13:8:   required from here
/home/lewis/gccdev/dev/rel/include/c++/13.0.0/ostream:754:5: error: no type
named ‘type’ in ‘struct std::enable_if<false, void>’
=============

I guess everyone will have a different option based on their workflow, but FWIW
I prefer the default output to be very short, along with an easy copy-pastable
option to see more. The intermediate option above which omits the source line
feels also like it still conveys enough information to me, say the user's code
with decltype etc is still printed because it's part of the diagnostic message
too. I'm happy to tweak it, also if you think it makes sense to handle say
system header locations or namespace std specially, I could do that too. I was
trying to avoid that and rather use the large number of overloads as an
indicator of "this is probably not worth printing", because special-casing the
standard library wouldn't help with other heavily templated libraries like
boost.