Allow overwriting read-only virtual properties in scope chains.

The previous behavior had an oversight: if you tried to set a variable with the same name as an in-scope property, it would always try to overwrite that property. This can fail silently and doesn't match with Flash Player behavior. Now, an attempt to overwrite a read-only property is instead correctly rejected so that it can be defined in local scope.

core/src/avm1/object.rs

@@ -108,6 +108,15 @@ impl<'gc> Property<'gc> {
             Property::Stored { attributes, .. } => !attributes.contains(DontEnum),
         }
     }
+
+    pub fn is_overwritable(&self) -> bool {
+        match self {
+            Property::Virtual {
+                attributes, set, ..
+            } => !attributes.contains(ReadOnly) && !set.is_none(),
+            Property::Stored { attributes, .. } => !attributes.contains(ReadOnly),
+        }
+    }
 }
 
 unsafe impl<'gc> gc_arena::Collect for Property<'gc> {
@@ -339,6 +348,13 @@ impl<'gc> Object<'gc> {
         self.values.contains_key(name)
     }
 
+    pub fn is_property_overwritable(&self, name: &str) -> bool {
+        self.values
+            .get(name)
+            .map(|p| p.is_overwritable())
+            .unwrap_or(false)
+    }
+
     pub fn get_keys(&self) -> Vec<String> {
         self.values
             .iter()

core/src/avm1/scope.rs

@@ -288,7 +288,7 @@ impl<'gc> Scope<'gc> {
         context: &mut UpdateContext<'_, 'gc, '_>,
         this: GcCell<'gc, Object<'gc>>,
     ) -> Result<Option<Value<'gc>>, Error> {
-        if self.locals().has_property(name) {
+        if self.locals().has_property(name) && self.locals().is_property_overwritable(name) {
             self.locals_mut(context.gc_context)
                 .set(name, value, avm, context, this)?;
             return Ok(None);