Constrained relational operators in line with P2944R3

.github/workflows/ci.yml

@@ -10,6 +10,7 @@ on:
 
 env:
   UBSAN_OPTIONS: print_stacktrace=1
+  ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
 
 jobs:
   posix:

doc/variant2/reference.adoc

@@ -845,6 +845,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() == std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: :: `v.index() == w.index() && get<I>(v) == get<I>(w)`, where `I`
   is `v.index()`.
 
@@ -855,6 +858,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() != std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: :: `!(v == w)`.
 
 ```
@@ -864,6 +870,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() < std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) < get<I>(w))`,
   where `I` is `v.index()`.
 
@@ -874,6 +883,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() < std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: :: `w < v`.
 
 ```
@@ -883,6 +895,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() \<= std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: :: `v.index() < w.index() || (v.index() == w.index() && get<I>(v) \<= get<I>(w))`,
   where `I` is `v.index()`.
 
@@ -893,6 +908,9 @@ template<class... T>
 [none]
 * {blank}
 +
+Constraints: :: `std::declval<const Ti&>() \<= std::declval<const Ti&>()` is a valid expression 
+  convertible to `bool` for all `Ti` in `T...`.
++
 Returns: ::
   `w \<= v`.
 

include/boost/variant2/variant.hpp

@@ -2103,6 +2103,45 @@ template<class... T> class variant: private detail::variant_ma_base<T...>
 namespace detail
 {
 
+namespace relops_constraints
+{
+
+#if !defined(BOOST_NO_SFINAE_EXPR) && \
+    !BOOST_WORKAROUND(BOOST_MSVC, < 1900) && !BOOST_WORKAROUND(BOOST_GCC, < 40900)
+
+template<class...> struct make_void { typedef void type; };
+
+#define BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( name, op ) \
+template<class T, class = void> struct name: std::false_type {}; \
+\
+template<class T> struct name<T, typename make_void< \
+    decltype( std::declval<const T&>() op std::declval<const T&>() )>::type>: \
+    std::is_convertible<decltype( std::declval<const T&>() op std::declval<const T&>() ), bool> \
+{ \
+};
+
+#else
+
+// non-expression-SFINAE fallback does not work with deleted relops: 
+// variant's relop instantiation fails instead, which is good enough
+
+struct not_comparable {};
+template<class U> not_comparable operator==( U const &, U const & );
+template<class U> not_comparable operator!=( U const &, U const & );
+template<class U> not_comparable operator< ( U const &, U const & );
+template<class U> not_comparable operator<=( U const &, U const & );
+
+#define BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( name, op ) \
+template<class T> struct name: std::is_convertible<decltype( std::declval<const T&>() op std::declval<const T&>() ), bool> \
+{ \
+};
+
+#endif
+
+BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( has_eq, == )
+
+} // namespace relops_constraints
+
 template<class... T> struct eq_L
 {
     variant<T...> const & v;
@@ -2116,14 +2155,22 @@ template<class... T> struct eq_L
 
 } // namespace detail
 
-template<class... T> constexpr bool operator==( variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_eq<T>...>::value>::type>
+constexpr bool operator==( variant<T...> const & v, variant<T...> const & w )
 {
     return v.index() == w.index() && mp11::mp_with_index<sizeof...(T)>( v.index(), detail::eq_L<T...>{ v, w } );
 }
 
 namespace detail
 {
 
+namespace relops_constraints
+{
+
+BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( has_ne, != )
+
+} // namespace relops_constraints
+
 template<class... T> struct ne_L
 {
     variant<T...> const & v;
@@ -2137,14 +2184,22 @@ template<class... T> struct ne_L
 
 } // namespace detail
 
-template<class... T> constexpr bool operator!=( variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_ne<T>...>::value>::type>
+constexpr bool operator!=( variant<T...> const & v, variant<T...> const & w )
 {
     return v.index() != w.index() || mp11::mp_with_index<sizeof...(T)>( v.index(), detail::ne_L<T...>{ v, w } );
 }
 
 namespace detail
 {
 
+namespace relops_constraints
+{
+
+BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( has_lt, < )
+
+} // namespace relops_constraints
+
 template<class... T> struct lt_L
 {
     variant<T...> const & v;
@@ -2158,19 +2213,29 @@ template<class... T> struct lt_L
 
 } // namespace detail
 
-template<class... T> constexpr bool operator<( variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_lt<T>...>::value>::type>
+constexpr bool operator<( variant<T...> const & v, variant<T...> const & w )
 {
     return v.index() < w.index() || ( v.index() == w.index() && mp11::mp_with_index<sizeof...(T)>( v.index(), detail::lt_L<T...>{ v, w } ) );
 }
 
-template<class... T> constexpr bool operator>(  variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_lt<T>...>::value>::type>
+constexpr bool operator>(  variant<T...> const & v, variant<T...> const & w )
 {
     return w < v;
 }
 
 namespace detail
 {
 
+namespace relops_constraints
+{
+
+BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT( has_le, <= )
+#undef BOOST_VARIANT2_DEFINE_RELOP_CONSTRAINT
+
+} // namespace relops_constraints
+
 template<class... T> struct le_L
 {
     variant<T...> const & v;
@@ -2184,12 +2249,14 @@ template<class... T> struct le_L
 
 } // namespace detail
 
-template<class... T> constexpr bool operator<=( variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_le<T>...>::value>::type>
+constexpr bool operator<=( variant<T...> const & v, variant<T...> const & w )
 {
     return v.index() < w.index() || ( v.index() == w.index() && mp11::mp_with_index<sizeof...(T)>( v.index(), detail::le_L<T...>{ v, w } ) );
 }
 
-template<class... T> constexpr bool operator>=( variant<T...> const & v, variant<T...> const & w )
+template<class... T,  class E = typename std::enable_if<mp11::mp_all<detail::relops_constraints::has_le<T>...>::value>::type>
+constexpr bool operator>=( variant<T...> const & v, variant<T...> const & w )
 {
     return w <= v;
 }

test/Jamfile

@@ -180,3 +180,5 @@ run variant_issue_55.cpp
     # clang-cl 32 bit fails with an assertion in mp_with_index, likely due to a codegen bug
     "<toolset>clang-win,<address-model>32:<build>no"
   ;
+
+run variant_constrained_relops.cpp ;