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This patch implements optional<T&> based on the P2988R12 paper, incorporating
corrections from LWG4300, LWG4304, and LWG3467. The resolution for LWG4015
is also extended to cover optional<T&>.
We introduce _M_fwd() helper, that is equivalent to operator*(), except that
it does not check non-empty precondition. It is used in to correctly propagate
the value during move construction from optional<T&>. This is necessary because
moving an optional<T&> must not move the contained object, which is the key
distinction between *std::move(opt) and std::move(*opt).
The implementation deviates from the standard by providing a separate std::swap
overload for std::optional<T&>, which simplifies preserving the resolution of
LWG2766.
This introduces a few changes to make_optional behavior (see included test):
* some previously valid uses of make_optional<T>({...}) (where T is not a
reference type) now become ill-formed (see optional/make_optional_neg.cc).
* make_optional<T&>(t) and make_optional<const T&>(ct), where decltype(t) is T&,
and decltype(ct) is const T& now produce optional<T&> and optional<const T&>
respectively, instead of optional<T>.
* a few other uses of make_optional<R> with reference type R are now ill-formed.
PR libstdc++/121748
libstdc++-v3/ChangeLog:
* include/bits/version.def: Bump value for optional,
* include/bits/version.h: Regenerate.
* include/std/optional (std::__is_valid_contained_type_for_optional):
Define.
(std::optional<T>): Use __is_valid_contained_type_for_optional.
(optional<T>(const optional<_Up>&), optional<T>(optional<_Up>&&))
(optional<T>::operator=(const optional<_Up>&))
(optional<T>::operator=(optional<_Up>&&)): Replacex._M_get() with
x._M_fwd(), and std::move(x._M_get()) with std::move(x)._M_fwd().
(optional<T>::and_then): Remove uncessary remove_cvref_t.
(optional<T>::_M_fwd): Define.
(std::optional<T&>): Define new partial specialization.
(std::swap(std::optional<T&>, std::optional<T&>)): Define.
(std::make_optional(_Tp&&)): Add non-type template parameter.
(std::make_optional): Use parenthesis to constructor optional.
(std::hash<optional<T>>): Add comment.
* testsuite/20_util/optional/make_optional-2.cc: Guarded not longer
working example.
* testsuite/20_util/optional/relops/constrained.cc: Expand test to
cover optionals of reference.
* testsuite/20_util/optional/requirements.cc: Ammend for
optional<T&>.
* testsuite/20_util/optional/requirements_neg.cc: Likewise.
* testsuite/20_util/optional/version.cc: Test new value of
__cpp_lib_optional.
* testsuite/20_util/optional/make_optional_neg.cc: New test.
* testsuite/20_util/optional/monadic/ref_neg.cc: New test.
* testsuite/20_util/optional/ref/access.cc: New test.
* testsuite/20_util/optional/ref/assign.cc: New test.
* testsuite/20_util/optional/ref/cons.cc: New test.
* testsuite/20_util/optional/ref/internal_traits.cc: New test.
* testsuite/20_util/optional/ref/make_optional/1.cc: New test.
* testsuite/20_util/optional/ref/make_optional/from_args_neg.cc:
New test.
* testsuite/20_util/optional/ref/make_optional/from_lvalue_neg.cc:
New test.
* testsuite/20_util/optional/ref/make_optional/from_rvalue_neg.cc:
New test.
* testsuite/20_util/optional/ref/monadic.cc: New test.
* testsuite/20_util/optional/ref/relops.cc: New test.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
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