The operator<=>(const optional<T>&, const U&) operator is supposed to be
constrained with three_way_comparable_with<U, T> so that it can only be
used when T and U are weakly-equality-comparable and also three-way
comparable.
Adding that constrain completely breaks std::optional comparisons,
because it causes constraint recursion. To avoid that, an additional
check that U is not a specialization of std::optional is needed. That
appears to be a defect in the standard and should be reported to LWG.
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
PR libstdc++/98842
* include/std/optional (operator<=>(const optional<T>& const U&)):
Add missing constraint and add workaround for template
recursion.
* testsuite/20_util/optional/relops/three_way.cc: Check that
type without equality comparison cannot be compared when wrapped
in std::optional.
adec148117