perf: use assignment to implement optional<> move assignment

Author: eric-wieser

src/runtime/optional.h

@@ -64,41 +64,54 @@ class optional {
     void emplace(Args&&... args) {
         if (m_some)
             m_value.~T();
-        m_some = true;
         new (&m_value) T(args...);
+        m_some = true;
     }
 
     optional& operator=(optional const & other) {
-        if (this == &other)
-            return *this;
-        if (m_some)
+        if (other.m_some) {
+            if (m_some) {
+                m_value = other.value;
+            } else {
+                new (&m_value) T(other.m_value);
+                m_some = true;
+            }
+        } else if (m_some) {
             m_value.~T();
-        m_some = other.m_some;
-        if (m_some)
-            new (&m_value) T(other.m_value);
+            m_some = false;
+        }
         return *this;
     }
     optional& operator=(optional && other) {
-        lean_assert(this != &other);
-        if (m_some)
+        if (other.m_some) {
+            if (m_some) {
+                m_value = std::move(other.value);
+            } else {
+                new (&m_value) T(std::move(other.m_value));
+                m_some = true;
+            }
+        } else if (m_some) {
             m_value.~T();
-        m_some = other.m_some;
-        if (m_some)
-            new (&m_value) T(std::move(other.m_value));
+            m_some = false;
+        }
         return *this;
     }
     optional& operator=(T const & other) {
-        if (m_some)
-            m_value.~T();
-        m_some = true;
-        new (&m_value) T(other);
+        if (m_some) {
+            m_value = other;
+        } else {
+            new (&m_value) T(other);
+            m_some = true;
+        }
         return *this;
     }
     optional& operator=(T && other) {
-        if (m_some)
-            m_value.~T();
-        m_some = true;
-        new (&m_value) T(std::move(other));
+        if (m_some) {
+            m_value = std::move(other);
+        } else {
+            new (&m_value) T(std::move(other));
+            m_some = true;
+        }
         return *this;
     }