Fix indeterminate ordering in typesToValues() (#4)

paulbiss · facebook-github-bot · commit 1174b9f2e593 · 2022-10-19T14:35:11.000-07:00
Summary: X-link: hhvm/hhvm-staging#4 We depend on the evaluation order of the lambda passed to typesToValues() (in particular we depend on it being called for each element type in the tuple in the order in which they appear in the tuple). In C++ the order in which the values of function parameters are resolved is indeterminate: > In a function call, value computations and side effects of the initialization of every parameter are indeterminately sequenced with respect to value computations and side effects of any other parameter. https://en.cppreference.com/w/cpp/language/eval_order This diff changes `typesToValuesImpl` to use a recursive definition that should have a well-defined order. Reviewed By: ricklavoie, alexeyt Differential Revision: D40489955 fbshipit-source-id: 4342dde6eeda4c1002795e4f7dc2ee41c9879152
diff --git a/hphp/util/extern-worker-detail.h b/hphp/util/extern-worker-detail.h
@@ -299,10 +299,23 @@ template <typename T> struct Tag { using Type = T; };
 // for each type in that tuple. The callable is called with the tuple
 // index as the first parameter, and Tag<T> as the second (where T is
 // the type at that tuple index).
-template <typename Tuple, typename F, size_t... Is>
+template <typename Tuple, typename F, typename... Args>
 auto typesToValuesImpl(F&& f,
-                       std::index_sequence<Is...>) {
-  return std::make_tuple(f(Is, Tag<std::tuple_element_t<Is, Tuple>>{})...);
+                       std::index_sequence<>,
+                       Args&&... args) {
+  return std::make_tuple(std::forward<Args>(args)...);
+}
+
+template <typename Tuple, typename F, typename... Args, size_t I, size_t... Is>
+auto typesToValuesImpl(F&& f,
+                       std::index_sequence<I, Is...>,
+                       Args&&... args) {
+  return typesToValuesImpl<Tuple>(
+    std::forward<F>(f),
+    std::index_sequence<Is...>{},
+    std::forward<Args>(args)...,
+    f(I, Tag<std::tuple_element_t<I, Tuple>>{})
+  );
 }
 
 template <typename Tuple, typename F>
