--- a/dom/base/WindowNamedPropertiesHandler.cpp
+++ b/dom/base/WindowNamedPropertiesHandler.cpp
@@ -157,17 +157,17 @@ bool
 WindowNamedPropertiesHandler::defineProperty(JSContext* aCx,
                                              JS::Handle<JSObject*> aProxy,
                                              JS::Handle<jsid> aId,
                                              JS::Handle<JS::PropertyDescriptor> aDesc,
                                              JS::ObjectOpResult &result) const
 {
   ErrorResult rv;
   rv.ThrowTypeError<MSG_DEFINEPROPERTY_ON_GSP>();
-  rv.MaybeSetPendingException(aCx);
+  MOZ_ALWAYS_TRUE(rv.MaybeSetPendingException(aCx));
   return false;
 }
 
 bool
 WindowNamedPropertiesHandler::ownPropNames(JSContext* aCx,
                                            JS::Handle<JSObject*> aProxy,
                                            unsigned flags,
                                            JS::AutoIdVector& aProps) const

--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -5464,17 +5464,17 @@ def getJSToNativeConversionInfo(type, de
               if (!JS_WrapValue(cx, &valueToResolve)) {
                 $*{exceptionCode}
               }
               binding_detail::FastErrorResult promiseRv;
               nsCOMPtr<nsIGlobalObject> global =
                 do_QueryInterface(promiseGlobal.GetAsSupports());
               if (!global) {
                 promiseRv.ThrowWithCustomCleanup(NS_ERROR_UNEXPECTED);
-                promiseRv.MaybeSetPendingException(cx);
+                MOZ_ALWAYS_TRUE(promiseRv.MaybeSetPendingException(cx));
                 $*{exceptionCode}
               }
               $${declName} = Promise::Resolve(global, cx, valueToResolve,
                                               promiseRv);
               if (promiseRv.MaybeSetPendingException(cx)) {
                 $*{exceptionCode}
               }
             }

--- a/dom/bindings/ErrorResult.h
+++ b/dom/bindings/ErrorResult.h
@@ -257,16 +257,17 @@ public:
   //
   // Note that a true return value does NOT mean there is now a pending
   // exception on aCx, due to uncatchable exceptions.  It should still be
   // considered equivalent to a JSAPI failure in terms of what callers should do
   // after true is returned.
   //
   // After this call, the TErrorResult will no longer return true from Failed(),
   // since the exception will have moved to the JSContext.
+  MOZ_MUST_USE
   bool MaybeSetPendingException(JSContext* cx)
   {
     WouldReportJSException();
     if (!Failed()) {
       return false;
     }
 
     SetPendingException(cx);

--- a/dom/bindings/ToJSValue.cpp
+++ b/dom/bindings/ToJSValue.cpp
@@ -51,20 +51,18 @@ ToJSValue(JSContext* aCx,
 bool
 ToJSValue(JSContext* aCx,
           ErrorResult& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
   MOZ_ASSERT(aArgument.Failed());
   MOZ_ASSERT(!aArgument.IsUncatchableException(),
              "Doesn't make sense to convert uncatchable exception to a JS value!");
-  DebugOnly<bool> throwResult = aArgument.MaybeSetPendingException(aCx);
-  MOZ_ASSERT(throwResult);
-  DebugOnly<bool> getPendingResult = JS_GetPendingException(aCx, aValue);
-  MOZ_ASSERT(getPendingResult);
+  MOZ_ALWAYS_TRUE(aArgument.MaybeSetPendingException(aCx));
+  MOZ_ALWAYS_TRUE(JS_GetPendingException(aCx, aValue));
   JS_ClearPendingException(aCx);
   return true;
 }
 
 bool
 ToJSValue(JSContext* aCx, Promise& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {

--- a/js/xpconnect/wrappers/AccessCheck.cpp
+++ b/js/xpconnect/wrappers/AccessCheck.cpp
@@ -315,17 +315,17 @@ AccessCheck::reportCrossOriginDenial(JSC
         message = NS_LITERAL_CSTRING("Permission denied to ") +
                   accessType +
                   NS_LITERAL_CSTRING(" property ") +
                   NS_ConvertUTF16toUTF8(propName) +
                   NS_LITERAL_CSTRING(" on cross-origin object");
     }
     ErrorResult rv;
     rv.ThrowDOMException(NS_ERROR_DOM_SECURITY_ERR, message);
-    rv.MaybeSetPendingException(cx);
+    MOZ_ALWAYS_TRUE(rv.MaybeSetPendingException(cx));
 }
 
 enum Access { READ = (1<<0), WRITE = (1<<1), NO_ACCESS = 0 };
 
 static void
 EnterAndThrowASCII(JSContext* cx, JSObject* wrapper, const char* msg)
 {
     JSAutoCompartment ac(cx, wrapper);