[llvm] [ADT] Avoid map storage for small SmallMapVector (PR #196473)

[Nikita Popov via llvm-commits]

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@@ -225,15 +286,67 @@ class MapVector {
   template <class Predicate> void remove_if(Predicate Pred);
 
 private:
+  template <typename LookupKeyT, typename StoredKeyT>
+  [[nodiscard]] static bool keyEqual(const LookupKeyT &LookupKey,
+                                     const StoredKeyT &StoredKey) {
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nikic wrote:

Why do we need this? We don't do this in SmallSetVector.

IMHO weird cases with mismatching DenseMapInfo and operator== should just opt-out by disabling the small-size optimization.

https://github.com/llvm/llvm-project/pull/196473