[llvm] MapVector: add C++17-style try_emplace and insert_or_assign (PR #71969)

[Fangrui Song via llvm-commits]

================
@@ -114,31 +114,56 @@ class MapVector {
     return Pos == Map.end()? ValueT() : Vector[Pos->second].second;
   }
 
-  std::pair<iterator, bool> insert(const std::pair<KeyT, ValueT> &KV) {
-    std::pair<KeyT, typename MapType::mapped_type> Pair = std::make_pair(KV.first, 0);
-    std::pair<typename MapType::iterator, bool> Result = Map.insert(Pair);
+  template <typename... Ts>
+  std::pair<iterator, bool> try_emplace(const KeyT &Key, Ts &&...Args) {
+    std::pair<typename MapType::iterator, bool> Result =
+        Map.insert(std::make_pair(Key, 0));
     auto &I = Result.first->second;
     if (Result.second) {
-      Vector.push_back(std::make_pair(KV.first, KV.second));
+      Vector.emplace_back(std::piecewise_construct, std::forward_as_tuple(Key),
+                          std::forward_as_tuple(std::forward<Ts>(Args)...));
       I = Vector.size() - 1;
       return std::make_pair(std::prev(end()), true);
     }
     return std::make_pair(begin() + I, false);
   }
-
-  std::pair<iterator, bool> insert(std::pair<KeyT, ValueT> &&KV) {
-    // Copy KV.first into the map, then move it into the vector.
-    std::pair<KeyT, typename MapType::mapped_type> Pair = std::make_pair(KV.first, 0);
-    std::pair<typename MapType::iterator, bool> Result = Map.insert(Pair);
+  template <typename... Ts>
+  std::pair<iterator, bool> try_emplace(KeyT &&Key, Ts &&...Args) {
+    std::pair<typename MapType::iterator, bool> Result =
+        Map.insert(std::make_pair(Key, 0));
----------------
MaskRay wrote:

If we use `std::move` here, `std::forward_as_tuple(static_cast<KeyT &&>(Key)),` below may referenced a moved-out `Key`.

An alternative is to use
```
Map.insert(std::make_pair(std::move(Key), 0)); // map insertion
...
std::forward_as_tuple(Result.first->first)  // vector insertion
```

but this does not save copy/move numbers.

https://github.com/llvm/llvm-project/pull/71969