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Question

The following code compiles before c++20:

#include <algorithm>
#include <vector>

struct Foo
{
};

struct FooArray
{
    std::vector<Foo> a;
};

bool operator<(const FooArray& a, const FooArray& b)
{
    return std::lexicographical_compare(std::begin(a.a), std::end(a.a), std::begin(b.a), std::end(b.a));
}

bool operator<(const Foo& a, const Foo& b)
{
    return false;
}

With c++20 enabled GCC 10 and 11 reject the code, but only when optimisations are enabled:

In file included from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/algorithm:61,
                 from <source>:1:
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/predefined_ops.h: In instantiation of 'constexpr bool __gnu_cxx::__ops::_Iter_less_iter::operator()(_Iterator1, _Iterator2) const [with _Iterator1 = const Foo*; _Iterator2 = const Foo*]':
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:1240:14:   required from 'constexpr bool std::__lexicographical_compare_impl(_II1, _II1, _II2, _II2, _Compare) [with _II1 = const Foo*; _II2 = const Foo*; _Compare = __gnu_cxx::__ops::_Iter_less_iter]'
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:1257:46:   required from 'static constexpr bool std::__lexicographical_compare<_BoolType>::__lc(_II1, _II1, _II2, _II2) [with _II1 = const Foo*; _II2 = const Foo*; bool _BoolType = false]'
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:1302:60:   required from 'constexpr bool std::__lexicographical_compare_aux(_II1, _II1, _II2, _II2) [with _II1 = const Foo*; _II2 = const Foo*]'
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:1605:48:   required from 'constexpr bool std::lexicographical_compare(_II1, _II1, _II2, _II2) [with _II1 = __gnu_cxx::__normal_iterator<const Foo*, std::vector<Foo> >; _II2 = __gnu_cxx::__normal_iterator<const Foo*, std::vector<Foo> >]'
<source>:15:40:   required from here
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/predefined_ops.h:43:23: error: no match for 'operator<' (operand types are 'const Foo' and 'const Foo')
   43 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
In file included from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/algorithm:61,
                 from <source>:1:
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_iterator.h:1097:5: note: candidate: 'template<class _IteratorL, class _IteratorR, class _Container> constexpr std::__detail::__synth3way_t<_IteratorR, _IteratorL> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_IteratorL, _Container>&, const __gnu_cxx::__normal_iterator<_IteratorR, _Container>&)' (reversed)
 1097 |     operator<=>(const __normal_iterator<_IteratorL, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_iterator.h:1097:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/algorithm:61,
                 from <source>:1:
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/predefined_ops.h:43:23: note:   'const Foo' is not derived from 'const __gnu_cxx::__normal_iterator<_IteratorL, _Container>'
   43 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
In file included from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:67,
                 from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/algorithm:61,
                 from <source>:1:
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_iterator.h:1114:5: note: candidate: 'template<class _Iterator, class _Container> constexpr std::__detail::__synth3way_t<_T1> __gnu_cxx::operator<=>(const __gnu_cxx::__normal_iterator<_Iterator, _Container>&, const __gnu_cxx::__normal_iterator<_Iterator, _Container>&)' (rewritten)
 1114 |     operator<=>(const __normal_iterator<_Iterator, _Container>& __lhs,
      |     ^~~~~~~~
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_iterator.h:1114:5: note:   template argument deduction/substitution failed:
In file included from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/stl_algobase.h:71,
                 from /opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/algorithm:61,
                 from <source>:1:
/opt/compiler-explorer/gcc-10.5.0/include/c++/10.5.0/bits/predefined_ops.h:43:23: note:   'const Foo' is not derived from 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>'
   43 |       { return *__it1 < *__it2; }
      |                ~~~~~~~^~~~~~~~
<source>:13:6: note: candidate: 'bool operator<(const FooArray&, const FooArray&)'
   13 | bool operator<(const FooArray& a, const FooArray& b)
      |      ^~~~~~~~
<source>:13:32: note:   no known conversion for argument 1 from 'const Foo' to 'const FooArray&'
   13 | bool operator<(const FooArray& a, const FooArray& b)
      |                ~~~~~~~~~~~~~~~~^
Compiler returned: 1

GCC 12 and later compile the code. MSVC always compiles the code, clang rejects it when c++20 is enabled.

https://godbolt.org/z/9ra6hn7c9

Which compiler is correct? Does bool operator<(const Foo& a, const Foo& b) need to be declared before calling std::lexicographical_compare? Is this a change in c++20 or did the code just happen to work in c++17 (I was surprised the code wasn’t causing problems in c++17 too)?

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Danh mục

Does operator < need to be declared before calling std::lexicographical_compare