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The implementation of less<> did not consider the possibility of t < u being
rewritten from overloaded operator<=>. This lead to situation when for t,u that:
* provide overload operator<=>, such that (t < u) is rewritten to (t <=> u) < 0,
* are convertible to pointers,
the expression std::less<>(t, u) would incorrectly result in call of
std::less<void*> on values converted to the pointers, instead of t < u.
The similar issues also occurred for greater<>, less_equal<>, greater_equal<>,
their range equivalents, and in three_way_compare for heterogeneous calls.
This patch addresses above, by also checking for free-functions and member
overloads of operator<=>, before falling back to pointer comparison. We do
not put any constraints on the return type of selected operator, in particular
in being one of the standard defined comparison categories, as the language
does not put any restriction of returned type, and if (t <=> u) is well
formed, (t op u) is interpreted as (t <=> u) op 0. If that later expression
is ill-formed, the expression using op also is (see included tests).
The relational operator rewrites try both order of arguments, t < u,
can be rewritten into operator<=>(t, u) < 0 or 0 < operator<=>(u, t), it
means that we need to test both operator<=>(T, U) and operator<=>(U, T)
if T and U are not the same types. This is now extracted into
__not_overloaded_spaceship helper concept, placed in <concepts>, to
avoid extending set of includes.
The compare_three_way functor defined in compare, already considers overloaded
operator<=>, however it does not consider reversed candidates, leading
to situation in which t <=> u results in 0 <=> operator<=>(u, t), while
compare_three_way{}(t, u) uses pointer comparison. This is also addressed by
using __not_overloaded_spaceship, that check both order of arguments.
Finally, as operator<=> is introduced in C++20, for std::less(_equal)?<>,
std::greater(_equal)?<>, we use provide separate __ptr_cmp implementation
in that mode, that relies on use of requires expression. We use a nested
requires clause to guarantee short-circuiting of their evaluation.
The operator() of aforementioned functors is reworked to use if constexpr,
in all standard modes (as we allow is as extension), eliminating the need
for _S_cmp function.
PR libstdc++/114153
libstdc++-v3/ChangeLog:
* include/bits/ranges_cmp.h (__detail::__less_builtin_ptr_cmp):
Add __not_overloaded_spaceship spaceship check.
* include/bits/stl_function.h (greater<void>::operator())
(less<void>::operator(), greater_equal<void>::operator())
(less_equal<void>::operator()): Implement using if constexpr.
(greater<void>::__S_cmp, less<void>::__S_cmp)
(greater_equal<void>::__ptr_comp, less_equal<void>::S_cmp):
Remove.
(greater<void>::__ptr_cmp, less<void>::__ptr_cmp)
(greater_equal<void>::__ptr_comp, less_equal<void>::ptr_cmp): Change
tostatic constexpr variable. Define in terms of requires expressions
and __not_overloaded_spaceship check.
* include/std/concepts: (__detail::__not_overloaded_spaceship):
Define.
* libsupc++/compare: (__detail::__3way_builtin_ptr_cmp): Use
__not_overloaded_spaceship concept.
* testsuite/20_util/function_objects/comparisons_pointer_spaceship.cc: New test.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>