QmlCompiler: Comment about not allowing modifications to return values

This trap is all too easy to forget about. Let's make sure the next
person thinking about it understands it.

Change-Id: Ib7bd566e1d8c4264a99ae59e79384c4e87581d43
Reviewed-by: Olivier De Cannière <[email protected]>
Reviewed-by: Sami Shalayel <[email protected]>
Reviewed-by: Ulf Hermann <[email protected]>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/qmlcompiler/qqmljscodegenerator.cpp b/src/qmlcompiler/qqmljscodegenerator.cpp
index c2aaeeec77..355b261241 100644
--- a/src/qmlcompiler/qqmljscodegenerator.cpp
+++ b/src/qmlcompiler/qqmljscodegenerator.cpp
@@ -1010,6 +1010,15 @@ void QQmlJSCodeGenerator::generateWriteBack(int registerIndex)
 
         const int lookupIndex = writeBack.resultLookupIndex();
         if (lookupIndex == -1) {
+            // This is essential for the soundness of the type system.
+            //
+            // If a value or a list is returned from a function, we cannot know
+            // whether it is a copy or a reference. Therefore, we cannot know whether
+            // we have to write it back and so we have to reject any write on it.
+            //
+            // Only if we are sure that the value is locally created we can be sure
+            // we don't have to write it back. In this latter case we could allow
+            // a modification that doesn't write back.
             reject(u"write-back of non-lookup"_s);
             break;
         }