[CSDiagnostics] Add assertion for trailing pack element

Author: li3zhen1

lib/Sema/CSDiagnostics.cpp

@@ -8202,7 +8202,9 @@ bool UnableToInferClosureReturnType::diagnoseAsError() {
 bool UnableToInferGenericPackElementType::diagnoseAsError() {
   auto *locator = getLocator();
   auto path = locator->getPath();
-  const auto packElementElt = (path.end() - 1)->getAs<LocatorPathElt::PackElement>();
+
+  auto packElementElt = (path.end() - 1)->getAs<LocatorPathElt::PackElement>();
+  assert(packElementElt && "Expected path to end with a pack element locator");
 
   if (isExpr<NilLiteralExpr>(getAnchor())) {
     // `nil` appears as an element of generic pack params, let's record a