Implement optional chaining

This change implements optional chaining (https://github.com/tc39/proposal-optional-chaining) by adding a new type of optional lookup with an offset to the end of a chain.
If `undefined` or `null` is encountered during an access marked as optional, we jump to that end offset.

Features:
- Full support for all kinds of optional chain
- With some codegen overhead but zero overhead during normal non-optional FieldMemberExpression resolution
- Properly retains this contexts and does not need to resolve anything twice (this has been an issue previously)
- No extra AST structures, just flags for existing ones

[ChangeLog][QtQml] Added support for optional chaining (https://github.com/tc39/proposal-optional-chaining)

Fixes: QTBUG-77926
Change-Id: I9a41cdc4ca272066c79c72b9b22206498a546843
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/qml/compiler/qv4bytecodegenerator_p.h b/src/qml/compiler/qv4bytecodegenerator_p.h
index 5244c443c4..4cd3b37ad3 100644
--- a/src/qml/compiler/qv4bytecodegenerator_p.h
+++ b/src/qml/compiler/qv4bytecodegenerator_p.h
@@ -196,6 +196,20 @@ QT_WARNING_POP
         return addJumpInstruction(data);
     }
 
+    Q_REQUIRED_RESULT Jump jumpOptionalLookup(int index)
+    {
+        Instruction::GetOptionalLookup data{};
+        data.index = index;
+        return addJumpInstruction(data);
+    }
+
+    Q_REQUIRED_RESULT Jump jumpOptionalProperty(int name)
+    {
+        Instruction::LoadOptionalProperty data{};
+        data.name = name;
+        return addJumpInstruction(data);
+    }
+
     void jumpStrictEqual(const StackSlot &lhs, const Label &target)
     {
         Instruction::CmpStrictEqual cmp;