From 70962f401c17151bbe4e9d95b651d1ad5d30206f Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109@gmail.com>
Date: Sun, 4 Feb 2024 19:20:19 +0800
Subject: [PATCH] Add comments

---
 clang/lib/Sema/SemaTemplateInstantiate.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 220883ea9395f0..8faa8c5b5257e9 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -243,6 +243,23 @@ Response HandleFunctionTemplateDecl(const FunctionTemplateDecl *FTD,
       if (NNS->isInstantiationDependent()) {
         if (const auto *TSTy = Ty->getAs<TemplateSpecializationType>()) {
           ArrayRef<TemplateArgument> Arguments = TSTy->template_arguments();
+          // Prefer template arguments from the injected-class-type if possible.
+          // For example,
+          // ```cpp
+          // template <class... Pack> struct S {
+          //   template <class T> void foo();
+          // };
+          // template <class... Pack> template <class T>
+          //           ^^^^^^^^^^^^^ InjectedTemplateArgs
+          //           They're of kind TemplateArgument::Pack, not of
+          //           TemplateArgument::Type.
+          // void S<Pack...>::foo() {}
+          //        ^^^^^^^
+          //        TSTy->template_arguments() (which are of PackExpansionType)
+          // ```
+          // This meets the contract in
+          // TreeTransform::TryExpandParameterPacks that the template arguments
+          // for unexpanded parameters should be of a Pack kind.
           if (TSTy->isCurrentInstantiation()) {
             auto *RD = TSTy->getCanonicalTypeInternal()->getAsCXXRecordDecl();
             if (ClassTemplateDecl *CTD = RD->getDescribedClassTemplate())