Bug 1615022 part 3. Disallow conversion of CopyableErrorResult to ErrorResult&. r=farre

This keeps us from passing it to functions that are planning to throw on the
ErrorResult and might throw a JS exception on it.

Differential Revision: https://phabricator.services.mozilla.com/D62633

--HG--
extra : moz-landing-system : lando

dom/bindings/ErrorIPCUtils.h

@@ -95,8 +95,10 @@ struct ParamTraits<mozilla::CopyableErrorResult> {
 
   static bool Read(const Message* aMsg, PickleIterator* aIter,
                    paramType* aResult) {
-    mozilla::ErrorResult& ref = static_cast<mozilla::ErrorResult&>(*aResult);
-    return ParamTraits<mozilla::ErrorResult>::Read(aMsg, aIter, &ref);
+    // We can't cast *aResult to ErrorResult&, so cheat and just cast
+    // to ErrorResult*.
+    return ParamTraits<mozilla::ErrorResult>::Read(
+        aMsg, aIter, reinterpret_cast<mozilla::ErrorResult*>(aResult));
   }
 };
 

dom/bindings/ErrorResult.h

@@ -792,6 +792,11 @@ class CopyableErrorResult
     return *this;
   }
 
+  // Disallow implicit converstion to non-const ErrorResult&, because that would
+  // allow people to throw exceptions on us while bypassing our checks for JS
+  // exceptions.
+  operator ErrorResult&() = delete;
+
   // Allow conversion to ErrorResult&& so we can move out of ourselves into
   // an ErrorResult.
   operator ErrorResult &&() && {

dom/clients/manager/ClientOpenWindowUtils.cpp

@@ -453,7 +453,7 @@ RefPtr<ClientOpPromise> ClientOpenWindowInCurrentProcess(
 #endif  // MOZ_WIDGET_ANDROID
 
   RefPtr<BrowsingContext> bc;
-  CopyableErrorResult rv;
+  ErrorResult rv;
   OpenWindow(aArgs, getter_AddRefs(bc), rv);
 
   nsCOMPtr<nsPIDOMWindowOuter> outerWindow(bc->GetDOMWindow());