QML: Warn about variables being used before their declaration

This collides with injected signal parameters. qmlcachegen cannot tell
those cases apart.

[ChangeLog][QML][Important Behavior Changes] QML warns about JavaScript
variables being used before their declaration now. This is almost always
a mistake. It is particularly dangerous in the presence of injected
signal parameters because qmlcachegen cannot identify a name collision
between an injected signal parameter and a variable being used before
its declaration. It therefore miscompiles such code. You can turn off
the deprecation warning using the "qt.qml.compiler" logging category.

Pick-to: 6.1
Task-number: QTBUG-89943
Change-Id: I8a9424ca8c6edd562402fe5c560ba7e8344b5585
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/qml/compiler/qv4compilerscanfunctions.cpp b/src/qml/compiler/qv4compilerscanfunctions.cpp
index 4c8f0d0658..c4f46ecf05 100644
--- a/src/qml/compiler/qv4compilerscanfunctions.cpp
+++ b/src/qml/compiler/qv4compilerscanfunctions.cpp
@@ -330,9 +330,13 @@ bool ScanFunctions::visit(PatternElement *ast)
     BoundNames names;
     ast->boundNames(&names);
 
-    QQmlJS::SourceLocation lastInitializerLocation = ast->lastSourceLocation();
-    if (_context->lastBlockInitializerLocation.isValid())
-        lastInitializerLocation = _context->lastBlockInitializerLocation;
+    QQmlJS::SourceLocation declarationLocation = ast->firstSourceLocation();
+    if (_context->lastBlockInitializerLocation.isValid()) {
+        declarationLocation.length = _context->lastBlockInitializerLocation.end()
+                - declarationLocation.offset;
+    } else {
+        declarationLocation.length = ast->lastSourceLocation().end() - declarationLocation.offset;
+    }
 
     for (const auto &name : qAsConst(names)) {
         if (_context->isStrict && (name.id == QLatin1String("eval") || name.id == QLatin1String("arguments")))
@@ -345,7 +349,7 @@ bool ScanFunctions::visit(PatternElement *ast)
             return false;
         }
         if (!_context->addLocalVar(name.id, ast->initializer ? Context::VariableDefinition : Context::VariableDeclaration, ast->scope,
-                                   /*function*/nullptr, lastInitializerLocation)) {
+                                   /*function*/nullptr, declarationLocation)) {
             _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Identifier %1 has already been declared").arg(name.id));
             return false;
         }