[gcc r10-9996] libstdc++: Constrain three-way comparison for std::optional [PR 98842]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:3a415b6a93765f29bffd0582a001bf03c4b93f3c

commit r10-9996-g3a415b6a93765f29bffd0582a001bf03c4b93f3c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Jun 7 13:02:15 2021 +0100

    libstdc++: Constrain three-way comparison for std::optional [PR 98842]
    
    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.
    
    (cherry picked from commit adec14811714e22a6c1f7f0199adc05370f0d8b0)

Diff:
---
 libstdc++-v3/include/std/optional                  |  7 +++++++
 .../testsuite/20_util/optional/relops/three_way.cc | 22 ++++++++++++++++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index e3fbe625f25..4975bc6f922 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1210,7 +1210,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return !__rhs || __lhs >= *__rhs; }
 
 #ifdef __cpp_lib_three_way_comparison
+  template<typename _Tp>
+    inline constexpr bool __is_optional_v = false;
+  template<typename _Tp>
+    inline constexpr bool __is_optional_v<optional<_Tp>> = true;
+
   template<typename _Tp, typename _Up>
+    requires (!__is_optional_v<_Up>)
+      && three_way_comparable_with<_Tp, _Up>
     constexpr compare_three_way_result_t<_Tp, _Up>
     operator<=>(const optional<_Tp>& __x, const _Up& __v)
     { return bool(__x) ? *__x <=> __v : strong_ordering::less; }
diff --git a/libstdc++-v3/testsuite/20_util/optional/relops/three_way.cc b/libstdc++-v3/testsuite/20_util/optional/relops/three_way.cc
index d7bd16bf32a..416e6a9b891 100644
--- a/libstdc++-v3/testsuite/20_util/optional/relops/three_way.cc
+++ b/libstdc++-v3/testsuite/20_util/optional/relops/three_way.cc
@@ -15,8 +15,8 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-options "-std=gnu++2a" }
-// { dg-do compile { target c++2a } }
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
 
 #include <optional>
 
@@ -74,3 +74,21 @@ test02()
   static_assert( nullopt <= O{} );
   static_assert( nullopt <= O{1} );
 }
+
+template<typename T>
+  concept has_spaceship = requires (const T& t) { t <=> t; };
+
+void
+test03()
+{
+  struct E
+  {
+    auto operator<=>(const E&) const { return std::strong_ordering::equal; }
+  };
+  static_assert( !std::three_way_comparable<E> ); // not equality comparable
+  using O = std::optional<E>;
+  static_assert( !std::three_way_comparable<O> );
+  static_assert( ! has_spaceship<O> ); // PR libstdc++/98842
+  struct U : O { };
+  static_assert( ! has_spaceship<U> );
+}