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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/92472] enhancement: 5 * constify some parameters
Date: Wed, 06 May 2020 15:30:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D92472

--- Comment #12 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #11)
> (In reply to David Binderman from comment #10)
> > I am still not sure if the new code is ok or not,
>=20
> Wasn't "This is 400% wrong" clear?

Here's the error it caused:

/home/jwakely/build/powerpc64le-unknown-linux-gnu/libstdc++-v3/include/para=
llel/multiway_merge.h:273:
error: no match for 'operator<' in '__seq2 < __seq0' (operand types are
'__gnu_parallel::_GuardedIterator<__gnu_cxx::__normal_iterator<short unsign=
ed
int*, std::vector<short unsigned int> >, std::less<short unsigned int> >' a=
nd
'__gnu_parallel::_GuardedIterator<__gnu_cxx::__normal_iterator<short unsign=
ed
int*, std::vector<short unsigned int> >, std::less<short unsigned int> >')
/home/jwakely/build/powerpc64le-unknown-linux-gnu/libstdc++-v3/include/para=
llel/multiway_merge.h:120:
note: candidate: 'bool
__gnu_parallel::operator<(__gnu_parallel::_GuardedIterator<__gnu_cxx::__nor=
mal_iterator<short
unsigned int*, std::vector<short unsigned int> >, std::less<short unsigned =
int>
>&, __gnu_parallel::_GuardedIterator<__gnu_cxx::__normal_iterator<short
unsigned int*, std::vector<short unsigned int> >, const std::less<short
unsigned int> >&)'
/home/jwakely/build/powerpc64le-unknown-linux-gnu/libstdc++-v3/include/para=
llel/multiway_merge.h:121:
note:   no known conversion for argument 2 from
'__gnu_parallel::_GuardedIterator<__gnu_cxx::__normal_iterator<short unsign=
ed
int*, std::vector<short unsigned int> >, std::less<short unsigned int> >' to
'__gnu_parallel::_GuardedIterator<__gnu_cxx::__normal_iterator<short unsign=
ed
int*, std::vector<short unsigned int> >, const std::less<short unsigned int>
>&'


And the reason it's wrong is that const Foo<T, U>& and Foo<T, const U>& are
completely different things. Your patch made *one* of the parameters Foo<T,
const U>& (which is wrong because it's never called with anything matching =
that
type) and left the other parameter unchanged (which is even more wrong, bec=
ause
now the operator can't be used to compare two objects of the same type).



( https://gcc.gnu.org/pipermail/gcc-patches/2020-May/545214.html=