Added more checks for optional serialization support.

source/handy_http_primitives/optional.d

@@ -100,6 +100,12 @@ struct Optional(T) {
             * Returns: An exception if one is thrown.
             */
             SerdeException deserializeFromAsdf(Asdf data) {
+                static if (!__traits(compiles, {T t; t = T.init;})) {
+                    return new SerdeException(
+                        "Cannot deserialize Optional that contains immutable or otherwise unassignable value. " ~
+                        "Ensure that the following code compiles: T t; t = T.init; for the given type T."
+                    );
+                } else {
                     if (data == null) {
                         this.isNull = true;
                         this.value = T.init;
@@ -109,6 +115,7 @@ struct Optional(T) {
                     }
                     return null;
                 }
+            }
 
             /**
             * Serializes an optional value to allow for the automatic serialization
@@ -183,5 +190,21 @@ unittest {
         assert(sa.b && sa.b.value == "hello world!");
         assert(sa.c && sa.c.value == 3.14f);
         assert(sa.d && sa.d.value == Sd(42, 67));
+
+        // Check that optionals containing immutable data aren't supported for serialization & deserialization:
+        struct Invalid {
+            immutable int x;
+        }
+        Optional!Invalid opt = Optional!(Invalid).of(Invalid(42));
+        static assert(!__traits(compiles, serializeToJson(opt)));
+        try {
+            deserialize!(Optional!(Invalid))(`{"x": 123}`);
+            assert(
+                false,
+                "Failed to ensure that an exception is thrown when attempting to deserialize an incompatible Optional."
+            );
+        } catch (SerdeException exc) {
+            // pass.
+        }
     }
 }