From ba64eaab043d2449da787963c181e46bbd2b74be Mon Sep 17 00:00:00 2001
From: Andreas Fertig <andy@cppinsights.io>
Date: Wed, 25 Jun 2025 17:47:42 +0200
Subject: [PATCH] Added support for P2662r3.

---
 CodeGenerator.cpp      | 17 +++++++++++++
 CodeGeneratorTypes.h   |  1 +
 tests/p2662Test.cpp    | 20 +++++++++++++++
 tests/p2662Test.expect | 58 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 96 insertions(+)
 create mode 100644 tests/p2662Test.cpp
 create mode 100644 tests/p2662Test.expect

diff --git a/CodeGenerator.cpp b/CodeGenerator.cpp
index b2e081c1..4ad899ef 100644
--- a/CodeGenerator.cpp
+++ b/CodeGenerator.cpp
@@ -2112,6 +2112,23 @@ void CodeGenerator::InsertArg(const CXXFoldExpr* stmt)
 }
 //-----------------------------------------------------------------------------
 
+void CodeGenerator::InsertArg(const PackIndexingExpr* stmt)
+{
+    if(stmt->isFullySubstituted()) {
+        const auto* constExpr = dyn_cast_or_null<ConstantExpr>(stmt->getIndexExpr());
+
+        mOutputFormatHelper.Append(BuildInternalVarName(GetName(*stmt->getPackDecl())),
+                                   constExpr->getAPValueResult().getInt());
+
+    } else {
+        mOutputFormatHelper.Append(GetName(*stmt->getPackDecl()), "...[");
+
+        InsertArg(stmt->getIndexExpr());
+        mOutputFormatHelper.Append("]");
+    }
+}
+//-----------------------------------------------------------------------------
+
 void CodeGenerator::InsertArg(const CXXInheritedCtorInitExpr* stmt)
 {
     const auto& constructorDecl = *stmt->getConstructor();
diff --git a/CodeGeneratorTypes.h b/CodeGeneratorTypes.h
index 91299e82..9aa07225 100644
--- a/CodeGeneratorTypes.h
+++ b/CodeGeneratorTypes.h
@@ -136,6 +136,7 @@ SUPPORTED_STMT(SourceLocExpr)
 SUPPORTED_STMT(CXXParenListInitExpr)
 SUPPORTED_STMT(CppInsightsCommentStmt)
 SUPPORTED_STMT(CXXPseudoDestructorExpr)
+SUPPORTED_STMT(PackIndexingExpr)
 
 #undef IGNORED_DECL
 #undef IGNORED_STMT
diff --git a/tests/p2662Test.cpp b/tests/p2662Test.cpp
new file mode 100644
index 00000000..3ca73191
--- /dev/null
+++ b/tests/p2662Test.cpp
@@ -0,0 +1,20 @@
+// cmdline:-std=c++26
+
+template<typename... T>
+constexpr auto first_plus_last(T... values) -> T...[0]
+{
+  return T...[0](values...[0] + values...[sizeof...(values) - 1]);
+}
+
+// first_plus_last(); // ill formed
+static_assert(first_plus_last(1, 2, 10) == 11);
+
+auto res = [](auto... pack) {
+  decltype(pack...[0])   x5;      // type is int
+  decltype((pack...[0])) x6{x5};  // type is int&
+
+  return 0;
+}(0, 3.14, 'c');
+
+int main() {}
+
diff --git a/tests/p2662Test.expect b/tests/p2662Test.expect
new file mode 100644
index 00000000..d6255a23
--- /dev/null
+++ b/tests/p2662Test.expect
@@ -0,0 +1,58 @@
+template<typename ... T>
+inline constexpr T...[0] first_plus_last(T... values)
+{
+  return T...[0](values...[0] + values...[sizeof...(values) - 1]);
+}
+
+/* First instantiated from: p2662Test.cpp:10 */
+#ifdef INSIGHTS_USE_TEMPLATE
+template<>
+inline constexpr int first_plus_last<int, int, int>(int __values0, int __values1, int __values2)
+{
+  return int(__values0 + __values2);
+}
+#endif
+
+
+/* PASSED: static_assert(first_plus_last(1, 2, 10) == 11); */
+
+
+class __lambda_12_12
+{
+  public: 
+  template<class ... type_parameter_0_0>
+  inline /*constexpr */ auto operator()(type_parameter_0_0... pack) const
+  {
+    decltype(pack...[0]) x5;
+    decltype((pack...[0])) x6 = {x5};
+    return 0;
+  }
+  
+  #ifdef INSIGHTS_USE_TEMPLATE
+  template<>
+  inline /*constexpr */ int operator()<int, double, char>(int __pack0, double __pack1, char __pack2) const
+  {
+    int x5;
+    int & x6 = {x5};
+    return 0;
+  }
+  #endif
+  
+  private: 
+  template<class ... type_parameter_0_0>
+  static inline /*constexpr */ auto __invoke(type_parameter_0_0... pack)
+  {
+    return __lambda_12_12{}.operator()<type_parameter_0_0... >(pack...);
+  }
+  
+  public:
+  // /*constexpr */ __lambda_12_12() = default;
+  
+} __lambda_12_12{};
+
+int res = __lambda_12_12.operator()(0, 3.1400000000000001, 'c');
+
+int main()
+{
+  return 0;
+}