Fix build by explicitly defining a move assignment operator

src/units/abilities.cpp

@@ -784,6 +784,14 @@ attack_type::specials_context_t::specials_context_t(attack_type::specials_contex
 	other.was_moved = true;
 }
 
+attack_type::specials_context_t& attack_type::specials_context_t::operator=(attack_type::specials_context_t&& other)
+{
+	// This ugly line calls into the default copy move assignment operator.
+	operator=(static_cast<attack_type::specials_context_t&>(other));
+	other.was_moved = true;
+	return *this;
+}
+
 /**
  * Calculates the number of attacks this weapon has, considering specials.
  * This returns two numbers because of the swarm special. The actual number of

src/units/attack_type.hpp

@@ -125,11 +125,14 @@ public:
 		/// Initialize weapon specials context for a pair of units
 		specials_context_t(const attack_type& weapon, unit_const_ptr self, const map_location& loc, bool attacking);
 		specials_context_t(const specials_context_t&) = delete;
+		// Default assignment is needed as a base for the move assignment
+		specials_context_t& operator=(const specials_context_t&) = default;
 		bool was_moved = false;
 	public:
 		// Destructor at least needs to be public for all this to work.
 		~specials_context_t();
 		specials_context_t(specials_context_t&&);
+		specials_context_t& operator=(specials_context_t&&);
 	};
 	// Set up a specials context.
 	// Usage: auto ctx = weapon.specials_context(...);