Fix type constraints

Source/Engine/Core/Types/Nullable.h

@@ -7,13 +7,19 @@
 /// <summary>
 /// Wrapper for a value type that can be assigned null, controlling the lifetime of the wrapped value.
 /// </summary>
+/// <typeparam name="T">
+/// The type of the wrapped value. It must be move-constructible but does not have to be copy-constructible. Value is never reassigned.
+/// </typeparam>
 template<typename T>
 struct Nullable
 {
 private:
+    struct Dummy { Dummy() {} };
+
     union
     {
-        T _value;
+        T     _value;
+        Dummy _dummy;
     };
     bool _hasValue;
 
@@ -28,12 +34,18 @@ private:
         }
     }
 
+    /// <summary>
+    /// <c>true</c> if the wrapped type is copy constructible.
+    /// </summary>
+    constexpr static bool IsCopyConstructible = TIsCopyConstructible<T>::Value;
+
 public:
     /// <summary>
     /// Initializes <see cref="Nullable{T}"/> by setting the wrapped value to null.
     /// </summary>
     Nullable()
-        : _hasValue(false)
+        : _dummy()
+        , _hasValue(false)
     {
         // Value is not initialized.
     }
@@ -47,6 +59,7 @@ public:
     /// Initializes <see cref="Nullable{T}"/> by copying the wrapped value.
     /// </summary>
     /// <NullableBase name="value">The initial wrapped value to be copied.</param>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     Nullable(const T& value)
         : _value(value)
         , _hasValue(true)
@@ -67,6 +80,7 @@ public:
     /// Initializes <see cref="Nullable{T}"/> by copying another <see cref="Nullable{T}"/>.
     /// </summary>
     /// <param name="other">The wrapped value to be copied.</param>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     Nullable(const Nullable& other)
         : _value(other._value)
         , _hasValue(other._hasValue)
@@ -91,6 +105,7 @@ public:
     /// <summary>
     /// Reassigns the wrapped value by copying.
     /// </summary>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     auto operator=(const T& value) -> Nullable&
     {
         KillOld();
@@ -117,6 +132,7 @@ public:
     /// <summary>
     /// Reassigns the wrapped value by copying another <see cref="Nullable{T}"/>.
     /// </summary>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     auto operator=(const Nullable& other) -> Nullable&
     {
         KillOld();
@@ -205,6 +221,7 @@ public:
     /// Sets the wrapped value by copying.
     /// </summary>
     /// <param name="value">The value to be copied.</param>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     FORCE_INLINE void SetValue(const T& value)
     {
         if (_hasValue)
@@ -232,6 +249,7 @@ public:
     /// If the wrapped value is not valid, sets it by copying. Otherwise, does nothing.
     /// </summary>
     /// <returns>True if the wrapped value was changed, otherwise false.</returns>
+    template<typename U = T, typename = typename TEnableIf<IsCopyConstructible>::Type>
     FORCE_INLINE bool TrySet(const T& value)
     {
         if (_hasValue)