Let StaticMeta have a deleted dtor

Summary: [Folly] Let `StaticMeta` have a deleted dtor - a compile-time enforcement that the dtor will not be invoked, to replace the existing runtime non-enforcement.

Reviewed By: elsteveogrande

Differential Revision: D9624686

fbshipit-source-id: 9185defbea343e4123e66c8d0d38f20c561aebea

folly/detail/ThreadLocalDetail.h

@@ -342,10 +342,6 @@ struct StaticMetaBase {
 
   StaticMetaBase(ThreadEntry* (*threadEntry)(), bool strict);
 
-  [[noreturn]] ~StaticMetaBase() {
-    folly::assume_unreachable();
-  }
-
   void push_back(ThreadEntry* t) {
     t->next = &head_;
     t->prev = head_.prev;
@@ -403,6 +399,9 @@ struct StaticMetaBase {
   ThreadEntry head_;
   ThreadEntry* (*threadEntry_)();
   bool strict_;
+
+ protected:
+  ~StaticMetaBase() {}
 };
 
 // Held in a singleton to track our global instances.
@@ -413,7 +412,7 @@ struct StaticMetaBase {
 // for threads that use ThreadLocalPtr objects collide on a lock inside
 // StaticMeta; you can specify multiple Tag types to break that lock.
 template <class Tag, class AccessMode>
-struct StaticMeta : StaticMetaBase {
+struct StaticMeta final : StaticMetaBase {
   StaticMeta()
       : StaticMetaBase(
             &StaticMeta::getThreadEntrySlow,
@@ -425,6 +424,8 @@ struct StaticMeta : StaticMetaBase {
         /*child*/ &StaticMeta::onForkChild);
   }
 
+  ~StaticMeta() = delete;
+
   static StaticMeta<Tag, AccessMode>& instance() {
     // Leak it on exit, there's only one per process and we don't have to
     // worry about synchronization with exiting threads.