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
