[clang] [Clang] Avoid quadratic pack-indexing instantiation (store only the selected element) (PR #213790)

[Younan Zhang via cfe-commits]

================
@@ -7167,6 +7173,43 @@ TreeTransform<Derived>::TransformPackIndexingType(TypeLocBuilder &TLB,
       SubtitutedTypes.push_back(Pack);
       continue;
     }
+    // Fast path: substitute only the selected element instead of all N. A
+    // pack-indexing type inside a pack expansion (`T...[Is]...`) is transformed
+    // once per outer element, so substituting the whole pack each time is
+    // O(N^2) in time and memory.
+    if (!RetainExpansion && Types.size() == 1 && IndexExpr.isUsable() &&
+        !IndexExpr.get()->isInstantiationDependent()) {
----------------
zyn0217 wrote:

Why does it have to live inside the loop of `Types` if we are dealing with cases where there's only a Type?

https://github.com/llvm/llvm-project/pull/213790