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, 27 insertions, 0 deletions

diff --git a/src/qml/jsruntime/qv4vme_moth.cpp b/src/qml/jsruntime/qv4vme_moth.cpp
index 9077529277..db353c1834 100644
--- a/src/qml/jsruntime/qv4vme_moth.cpp
+++ b/src/qml/jsruntime/qv4vme_moth.cpp
@@ -700,6 +700,18 @@ QV4::ReturnedValue VME::interpret(JSTypesStackFrame *frame, ExecutionEngine *eng
         CHECK_EXCEPTION;
     MOTH_END_INSTR(LoadProperty)
 
+    MOTH_BEGIN_INSTR(LoadOptionalProperty)
+        STORE_IP();
+        STORE_ACC();
+        if (accumulator.isNullOrUndefined()) {
+            acc = Encode::undefined();
+            code += offset;
+        } else {
+            acc = Runtime::LoadProperty::call(engine, accumulator, name);
+        }
+        CHECK_EXCEPTION;
+    MOTH_END_INSTR(LoadOptionalProperty)
+
     MOTH_BEGIN_INSTR(GetLookup)
         STORE_IP();
         STORE_ACC();
@@ -718,6 +730,21 @@ QV4::ReturnedValue VME::interpret(JSTypesStackFrame *frame, ExecutionEngine *eng
         CHECK_EXCEPTION;
     MOTH_END_INSTR(GetLookup)
 
+    MOTH_BEGIN_INSTR(GetOptionalLookup)
+        STORE_IP();
+        STORE_ACC();
+
+        QV4::Lookup *l = function->executableCompilationUnit()->runtimeLookups + index;
+
+        if (accumulator.isNullOrUndefined()) {
+            acc = Encode::undefined();
+            code += offset;
+        } else {
+            acc = l->getter(l, engine, accumulator);
+        }
+    CHECK_EXCEPTION;
+    MOTH_END_INSTR(GetOptionalLookup)
+
     MOTH_BEGIN_INSTR(StoreProperty)
         STORE_IP();
         STORE_ACC();