Fix feature testing and add missing guards on other operator !=

include/ddc/aligned_allocator.hpp

@@ -61,10 +61,13 @@ constexpr bool operator==(AlignedAllocator<T, NT> const&, AlignedAllocator<U, NU
     return std::is_same_v<AlignedAllocator<T, NT>, AlignedAllocator<U, NU>>;
 }
 
+#if !defined(__cpp_impl_three_way_comparison)
+// In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
 template <class T, std::size_t NT, class U, std::size_t NU>
 constexpr bool operator!=(AlignedAllocator<T, NT> const&, AlignedAllocator<U, NU> const&) noexcept
 {
     return !std::is_same_v<AlignedAllocator<T, NT>, AlignedAllocator<U, NU>>;
 }
+#endif
 
 } // namespace ddc

include/ddc/detail/tagged_vector.hpp

@@ -319,12 +319,15 @@ class TaggedVector : public TaggedVectorConversionOperators<TaggedVector<Element
         return ((m_values[type_seq_rank_v<Tags, tags_seq>] == rhs.template get<Tags>()) && ...);
     }
 
+#if !defined(__cpp_impl_three_way_comparison)
+    // In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
     template <class OElementType, class... OTags>
     KOKKOS_FUNCTION constexpr bool operator!=(
             TaggedVector<OElementType, OTags...> const& rhs) const noexcept
     {
         return !(*this == rhs);
     }
+#endif
 
     template <class QueryTag>
     KOKKOS_FUNCTION constexpr ElementType& get() noexcept

include/ddc/discrete_domain.hpp

@@ -109,9 +109,8 @@ class DiscreteDomain
         return m_element_begin == other.m_element_begin && m_element_end == other.m_element_end;
     }
 
-#if __cplusplus <= 201703L
-    // Shall not be necessary anymore in C++20
-    // `a!=b` shall be translated by the compiler to `!(a==b)`
+#if !defined(__cpp_impl_three_way_comparison)
+    // In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
     template <class... ODims>
     KOKKOS_FUNCTION constexpr bool operator!=(DiscreteDomain<ODims...> const& other) const
     {
@@ -293,9 +292,8 @@ class DiscreteDomain<>
         return true;
     }
 
-#if __cplusplus <= 201703L
-    // Shall not be necessary anymore in C++20
-    // `a!=b` shall be translated by the compiler to `!(a==b)`
+#if !defined(__cpp_impl_three_way_comparison)
+    // In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
     KOKKOS_FUNCTION constexpr bool operator!=(DiscreteDomain const& other) const
     {
         return !(*this == other);
@@ -599,12 +597,15 @@ struct DiscreteDomainIterator
         return xx.m_value == yy.m_value;
     }
 
+#if !defined(__cpp_impl_three_way_comparison)
+    // In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
     friend KOKKOS_FUNCTION constexpr bool operator!=(
             DiscreteDomainIterator const& xx,
             DiscreteDomainIterator const& yy)
     {
         return xx.m_value != yy.m_value;
     }
+#endif
 
     friend KOKKOS_FUNCTION constexpr bool operator<(
             DiscreteDomainIterator const& xx,

include/ddc/discrete_element.hpp

@@ -326,13 +326,16 @@ KOKKOS_FUNCTION constexpr bool operator==(
     return ((lhs.template uid<Tags>() == rhs.template uid<Tags>()) && ...);
 }
 
+#if !defined(__cpp_impl_three_way_comparison)
+// In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
 template <class... Tags, class... OTags>
 KOKKOS_FUNCTION constexpr bool operator!=(
         DiscreteElement<Tags...> const& lhs,
         DiscreteElement<OTags...> const& rhs) noexcept
 {
     return !(lhs == rhs);
 }
+#endif
 
 template <class Tag>
 KOKKOS_FUNCTION constexpr bool operator<(

include/ddc/discrete_vector.hpp

@@ -306,11 +306,14 @@ class DiscreteVector : public detail::DiscreteVectorConversionOperators<Discrete
         return ((m_values[type_seq_rank_v<Tags, tags_seq>] == rhs.template get<Tags>()) && ...);
     }
 
+#if !defined(__cpp_impl_three_way_comparison)
+    // In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
     template <class... OTags>
     KOKKOS_FUNCTION constexpr bool operator!=(DiscreteVector<OTags...> const& rhs) const noexcept
     {
         return !(*this == rhs);
     }
+#endif
 
     template <class QueryTag>
     KOKKOS_FUNCTION constexpr DiscreteVectorElement& get() noexcept

include/ddc/kokkos_allocator.hpp

@@ -70,11 +70,14 @@ constexpr bool operator==(KokkosAllocator<T, MST> const&, KokkosAllocator<U, MSU
     return std::is_same_v<KokkosAllocator<T, MST>, KokkosAllocator<U, MSU>>;
 }
 
+#if !defined(__cpp_impl_three_way_comparison)
+// In C++20, `a!=b` shall be automatically translated by the compiler to `!(a==b)`
 template <class T, class MST, class U, class MSU>
 constexpr bool operator!=(KokkosAllocator<T, MST> const&, KokkosAllocator<U, MSU> const&) noexcept
 {
     return !std::is_same_v<KokkosAllocator<T, MST>, KokkosAllocator<U, MSU>>;
 }
+#endif
 
 template <class T>
 using DeviceAllocator = KokkosAllocator<T, Kokkos::DefaultExecutionSpace::memory_space>;